Eliminate nullability errors from :sdks:java:extensions:sql

[damccorm]

Just edit build.gradle and set enableChecker: true and fix some errors!

Imported from Jira BEAM-10497. Original Jira may contain additional context.
Reported by: kenn.
This issue has child subcomponents which were not migrated over. See the original Jira for more information.
Subtask of issue #20497