Treat empty blocks with MinItems == 0 as optional

[ovandriyanov]

This patch allows for generating documentation for those providers which have empty objects in their schema, i.e, the objects with no attributes and no nested blocks.

Terraform allows providers to have empty objects in their schema, but currently terraform-plugin-docs does not recognize such empty fields neither as required, nor optional, nor read-only, and thus cannot generate docs for such providers and raises an error like the following:

no match for empty_block_field, this can happen if you have incompatible schema
defined, for example an optional block where all the child attributes are
computed, in which case the block itself should also be marked computed

Fix it by handling such empty blocks explicitly.

Closes #100

[hashicorp-cla]

All committers have signed the CLA.

[GennadySpb]

@bflad Please check this PR: I got same issue as #100

[detro]

Sorry, erroneous approval.

This is good, but we need BUG FIXES entry added to the CHANGELOG

[detro]

Sorry, I think I should have been more explicit.

The changelog format is defined here https://www.terraform.io/plugin/sdkv2/best-practices/versioning#changelog-specification .

Your change will be part of 0.8.0.

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