chore: [do not merge] trying jspecify Null annotation

[suztomo]

Experimenting the new Nullness annotations from https://jspecify.dev/ project.

b/341380807

My memo: https://docs.google.com/document/d/1M7eg5XUk-cIezH_bt9n_ssAB2U5FZUnrnZi5MkY-Cx8

[sonarqubecloud[bot]]

Quality Gate passed for 'gapic-generator-java-root'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[sonarqubecloud[bot]]

Quality Gate passed for 'java_showcase_integration_tests'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud