Enable a static code analyzer for null-checking and other common issues

[bashir2]

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We have experimented with some static code analyzers like SonarQube (e.g., in #704, #706, #707) but these are not adequate, especially for catching NullPointerExceptions. Also we have some @Nullable annotations throughout the code but we don't have a robust null-check static code analyzer that can benefit from these annotations. With release 1.0.0 of Jspecify it makes sense to move our null-check annotations to Jspecify and enable a tool like NullAway on our code. This requires Error Prone which will help catching more issues at compile time.

An early experiment shows over 370 ErrorProne/NullAway warnings, so we should probably fix this issue in multiple phases (e.g., by module).

[ndegwamartin]

Here are the total ErrorProne/Nullaway/JavaStyleGuide static code analyzer warnings broken down by module (462 total).

------------ Pipelines ------------

Pipelines Common Module

• [x] Implementation code - 85 Warnings
• [ ] Test code - 72 Warnings

Pipelines Batch Module

• [x] Implementation code - 50 Warnings
• [x] Test code - 59 Warnings

Pipelines Controller Module

• [x] Implementation code - 97 Warnings
• [x] Test code - 8 Warnings

------------ Bunsen ------------

Bunsen Core Module

• [x] Implementation code - 33 Warnings
• [x] Test code - 6 Warnings

Bunsen Core R4 Module

• [x] Implementation code - 4 Warnings
• [x] Test code - 2 Warnings

Bunsen Core STU3 Module

• [x] Implementation code - 3 Warnings
• [x] Test code - 0 Warnings

Bunsen Avro Module

• [x] Implementation code - 30 Warnings
• [x] Test code - 13 Warnings

[bashir2]

Thanks @ndegwamartin for the breakdown. It would be nice to see how many of the production code warnings are from NullAway.

[ndegwamartin]

Thanks @ndegwamartin for the breakdown. It would be nice to see how many of the production code warnings are from NullAway.

Check out the comment here - https://github.com/google/fhir-data-pipes/issues/1474#issuecomment-3357265412

[bashir2]

Reopening this until all sub-issues are resolved.