Add java nullability annotations

[nvervelle]

Motivations:
Have generated clients annotated for nullability to be able to check code using them with tools like NullAway (to close #20804)

Modifications:

• First commit :
  • Adapt api templates to use the nullability template
  • Adapt tests
• Second commit :
  • Modifications from export_docs_generators.sh

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[martin-mfg]

Hi @nvervelle, thanks for the PR! I have only one tiny little remark - please see below.

[martin-mfg]

This assertion is anyway covered by the line below, right?

[nvervelle]

Hi @martin-mfg, yes, this was temporary test code to find more easily what annotations were used in each case because TestUtils.assertFileContains() doesn't output the file contents if it fails.

It's fixed.

[nvervelle]

Hi @martin-mfg , any hope of merging this PR in a near future ?

[martin-mfg]

Hi, this is up to @wing328.

[nvervelle]

Hi @wing328 : is the PR ok for you and can it be merged ?

[wing328]

@nvervelle thanks for the PR. Just merged it.

Have a good day.