Fix string .equals() to avoid NPE

[andreamlin]

Fixes googleapis/google-cloud-java#4320.

Currently, we are calling fieldName.equals("someResource") which will throw a NPE when variable fieldName is null.
Avoid the NPE by calling .equals() on the constant string instead.

[vam-google]

LGTM. Good fix and good practice in general (when compare to string literal call equals on the literal, not on the variable).

[codecov]

Codecov Report

Merging #2509 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master   #2509      +/-   ##
===========================================
- Coverage      86.8%   86.8%   -0.01%     
+ Complexity     5388    5385       -3     
===========================================
  Files           460     460              
  Lines         21261   21255       -6     
  Branches       2315    2315              
===========================================
- Hits          18456   18450       -6     
  Misses         1974    1974              
  Partials        831     831

Impacted Files	Coverage Δ	Complexity Δ
...m/google/api/codegen/config/LongRunningConfig.java	46.73% <0%> (-2.75%)	9% <0%> (-2%)
.../java/com/google/api/codegen/util/ProtoParser.java	72.58% <0%> (-0.22%)	45% <0%> (-1%)

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[codecov]

Codecov Report

Merging #2509 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master   #2509      +/-   ##
===========================================
- Coverage      86.8%   86.8%   -0.01%     
+ Complexity     5388    5385       -3     
===========================================
  Files           460     460              
  Lines         21261   21255       -6     
  Branches       2315    2315              
===========================================
- Hits          18456   18450       -6     
  Misses         1974    1974              
  Partials        831     831

Impacted Files	Coverage Δ	Complexity Δ
...m/google/api/codegen/config/LongRunningConfig.java	46.73% <0%> (-2.75%)	9% <0%> (-2%)
.../java/com/google/api/codegen/util/ProtoParser.java	72.58% <0%> (-0.22%)	45% <0%> (-1%)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a7708d8...17c4e8a. Read the comment docs.