Adds a check to make sure that the input cannot be null

[thijsoo]

Context

• We want to fix a bug that when you have bad data in the database saving a term can cause a fatal error.

Summary

This PR can be summarized in the following changelog entry:

• Fixes a bug where a error could occur when saving a taxonomy under circumstances that haven't been discovered.

Relevant technical choices:

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

I could not figure out how to test this without changing the code midway through execution with Xdebug. But since this occurs on yoast.com make sure to test this RC there to verify it all works.

For devs:

• Make sure premium is enabled.
• add two taxonomies (for example categories).
• For both of them add a Related keyphrase and synonym.
• Make sure both are saved.
• In the code place a breakpoint in /inc/options/class-wpseo-taxonomy-meta.php line238
• Without this PR. save a term and you should hit your breakpoint. hit Step over once and go into the Console.
• Add the following code: $input = null this simulates the json_decode failing.
• Hit Resume program and you should get a fatal.
• With this PR.
• save a term and you should hit your breakpoint. Look at the $input variable and fine the contents of your term by hitting Resume program untill you find your input.
• hit Step over once and go into the Console.
• Add the following code: $input = null this simulates the json_decode failing.
• Hit Resume program and you should NOT get a fatal.

Relevant test scenarios

• Changes should be tested with the browser console open
• Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies
• Changes should be tested on different editors (Default Block/Gutenberg/Classic/Elementor/other)
• Changes should be tested on different browsers
• Changes should be tested on multisite

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

QA can test this PR by following these steps:

• As mentioned above you need Xdebug to follow the code so test this on yoast.com staging.
• you should not get a fatal when you save a term, with Premium enabled (ask Rolf for replication steps of the original issue)

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

• Saving Keyphrase Synonyms for taxnomies.

Other environments

• This PR also affects Shopify. I have added a changelog entry starting with [shopify-seo], added test instructions for Shopify and attached the Shopify label to this PR.
• This PR also affects Yoast SEO for Google Docs. I have added a changelog entry starting with [yoast-doc-extension], added test instructions for Yoast SEO for Google Docs and attached the Google Docs Add-on label to this PR.

Documentation

• I have written documentation for this change. For example, comments in the Relevant technical choices, comments in the code, documentation on Confluence / shared Google Drive / Yoast developer portal, or other.

Quality assurance

• I have tested this code to the best of my abilities.
• During testing, I had activated all plugins that Yoast SEO provides integrations for.
• I have added unit tests to verify the code works as intended.
• If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched off.
• I have written this PR in accordance with my team's definition of done.
• I have checked that the base branch is correctly set.
• I have run grunt build:images and commited the results, if my PR introduces new images or SVGs.

Innovation

• No innovation project is applicable for this PR.
• This PR falls under an innovation project. I have attached the innovation label.
• I have added my hours to the WBSO document.

Fixes #

[coveralls]

Pull Request Test Coverage Report for Build 990db2b2719804cfcea88a6e3bd7163fdca8cdcd

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.5%) to 53.183%

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Totals
Change from base Build b51d845f89fe206089fb5db4f0ce36d8be6498bf:	-0.5%
Covered Lines:	33226
Relevant Lines:	62468

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💛 - Coveralls

[leonidasmi]

@thijsoo I pushed a quick fix of the PHPCS here, can I get a thumbs up?

[leonidasmi]

CR + Acceptance is ✅

For PHP 7.4, this is changing the behavior, because this is just a warning for that version, but I think we're ok with that since the value stays false-ish in the DB (also considering that we haven't been able to reproduce having null for a value there)