Fixes: Write Tests for graphql_config.dart

[AyushRaghuvanshi]

What kind of change does this PR introduce?

Issue Number: #2159

Fixes #2159

Did you add tests for your changes?

Yes

If relevant, did you update the documentation?

Yes

Summary
Upon Checking the code coverage this MockHttpClient was missing the test. I wrote a test for it. Since the endpoint does not exist and MockHttpClient is designed to throw 500. We always receive null in the data. For which I have tested it out.

Does this PR introduce a breaking change?
No.

Have you read the contributing guide?

Yes

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9749844) 88.27% compared to head (102aed6) 92.23%.
Report is 19 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2298      +/-   ##
===========================================
+ Coverage    88.27%   92.23%   +3.95%     
===========================================
  Files          155      157       +2     
  Lines         7352     7508     +156     
===========================================
+ Hits          6490     6925     +435     
+ Misses         862      583     -279     

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