Fix variablesJSONString store default for GraphQL page

[dihmeetree]

I don't have any variables set, and when running a mutation; I get the following error:

Error SyntaxError: Unexpected end of JSON input
    at JSON.parse (<anonymous>)
    at VueComponent._callee$ (graphql.vue?f047:505)
    at tryCatch (runtime.js?96cf:45)
    at Generator.invoke [as _invoke] (runtime.js?96cf:271)
    at Generator.prototype.<computed> [as next] (runtime.js?96cf:97)
    at asyncGeneratorStep (asyncToGenerator.js?1da1:3)
    at _next (asyncToGenerator.js?1da1:25)
    at eval (asyncToGenerator.js?1da1:32)
    at new Promise (<anonymous>)
    at VueComponent.eval (asyncToGenerator.js?1da1:21)

This error is caused by Postwoman trying to JSON parse an empty string, which is set as the default.
https://github.com/liyasthomas/postwoman/blob/f690ea01c4319d57ea311261c9857faf4a8f6baa/pages/graphql.vue#L505

Setting the variablesJSONString variable to the correct default ({}) fixes this issue :)

Before:

After:

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[TravisBuddy]

Hey @dmitryyankowski,
Your changes look good to me!

View build log

TravisBuddy Request Identifier: d1c196b0-56a1-11ea-8ad8-19cca21d7143