Fix nullness in BigQueryIO

[kennknowles]

BigQueryIO was identified as an area with a large amount of nullness errors and suppressions. This PR removes class level suppressions and fixes errors that were uncovered.

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[codecov]

Codecov Report

❌ Patch coverage is 77.12032% with 116 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.24%. Comparing base (87e31bd) to head (560b695).
⚠️ Report is 17 commits behind head on master.

Files with missing lines	Patch %	Lines
...eam/sdk/io/gcp/bigquery/BigQueryIOTranslation.java	64.49%	27 Missing and 22 partials ⚠️
...rg/apache/beam/sdk/io/gcp/bigquery/BigQueryIO.java	88.15%	10 Missing and 8 partials ⚠️
...igQueryStorageWriteApiSchemaTransformProvider.java	73.33%	5 Missing and 7 partials ⚠️
...apache/beam/sdk/io/gcp/bigquery/BigQueryUtils.java	86.36%	8 Missing and 1 partial ⚠️
...beam/sdk/io/gcp/bigquery/BigQueryServicesImpl.java	50.00%	5 Missing and 2 partials ⚠️
...ers/BigQueryDirectReadSchemaTransformProvider.java	70.00%	2 Missing and 4 partials ⚠️
...ery/BigQueryExportReadSchemaTransformProvider.java	73.33%	0 Missing and 4 partials ⚠️
...m/sdk/io/gcp/bigquery/UpdateSchemaDestination.java	50.00%	1 Missing and 3 partials ⚠️
...ders/BigQueryFileLoadsSchemaTransformProvider.java	69.23%	1 Missing and 3 partials ⚠️
...ache/beam/sdk/io/gcp/bigquery/BigQueryHelpers.java	75.00%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #39893      +/-   ##
============================================
- Coverage     58.65%   57.24%   -1.42%     
+ Complexity    15190     3661   -11529     
============================================
  Files          2781     1195    -1586     
  Lines        277838   193532   -84306     
  Branches      12274     3832    -8442     
============================================
- Hits         162954   110778   -52176     
+ Misses       108501    79174   -29327     
+ Partials       6383     3580    -2803     

Flag	Coverage Δ
java	72.27% <77.12%> (+7.57%)	⬆️

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[github-actions]

Assigning reviewers:

R: @ahmedabu98 for label java.

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[kennknowles]

Added @Pure annotations to getters to reduce the amount of noise from temporary locals. (without @Pure checkerframework correctly assumes that a getter method could modify object state). I will let all the tests re-run to green like last time before I merge.