Fixed parsing issue in Utf8GraphQLRequestParser#ParseStringOrNull

[glen-84]

Summary of the changes (Less than 80 chars)

• Fixed parsing issue in Utf8GraphQLRequestParser#ParseStringOrNull.

Closes #7664

---

This fixes the main issue, but IMO ParseStringOrNull shouldn't return null for an empty string.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.94%. Comparing base (02b1ca8) to head (0d843ff).
Report is 28 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7703      +/-   ##
==========================================
- Coverage   73.95%   73.94%   -0.02%     
==========================================
  Files        2917     2917              
  Lines      151116   151117       +1     
  Branches    17560    17560              
==========================================
- Hits       111753   111737      -16     
- Misses      33507    33525      +18     
+ Partials     5856     5855       -1     

Flag	Coverage Δ
unittests	73.94% <ø> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

---

🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests
• JS Bundle Analysis - Avoid shipping oversized bundles