fix: add default for no gql config case

[phani-srikar]

Description of changes

Adds a default return if the codegen config file does not exist to avoid breaking the pre/post push flows.

Issue #, if available

Description of how you validated changes

Added unit test.
Sample JS app.

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• Breaking changes to existing customers are released behind a feature flag or major version update
• Changes are tested using sample applications for all relevant platforms (iOS/android/flutter/Javascript) that use the feature added/modified

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #600 (a9ac730) into main (2d17486) will increase coverage by 0.06%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main     #600      +/-   ##
==========================================
+ Coverage   85.85%   85.92%   +0.06%     
==========================================
  Files         152      152              
  Lines        7487     7493       +6     
  Branches     1957     1959       +2     
==========================================
+ Hits         6428     6438      +10     
+ Misses        963      962       -1     
+ Partials       96       93       -3     

Impacted Files	Coverage Δ
...codegen/src/codegen-config/AmplifyCodeGenConfig.js	46.98% <100.00%> (+9.32%)	⬆️

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