feat: Prevent migration of singletonMap with null value or null key

[ZzetT]

What's changed?

The rule org.openrewrite.java.migrate.util.MigrateCollectionsSingletonMap also migrated the following cases:

Collections.singletonMap(null, "foo")
Collections.singletonMap("bar", null)

to

Map.of(null, "foo")
Map.of("bar", null)

which leads to a NullPointerException

What's your motivation?

There is a closed issue #571 where it was even mentioned in the comments to apply the same to maps, but the changes were not done in the previous PR.

Checklist

• I've added unit tests to cover both positive and negative cases
• I've read and applied the recipe conventions and best practices
• I've used the IntelliJ IDEA auto-formatter on affected files

[timtebeek]

Thanks for taking this on @ZzetT ! I've applied a small polish for a recent addition; if you recall where you copied that pattern from we can do the same there.

[ZzetT]

Thanks @timtebeek for the review and the quick merge.
I just wanted to add that we could applied your pattern also to MigrateCollectionsSingletonList and MigrateCollectionsSingletonSet, but I was a few seconds to late with my commit ;)

[timtebeek]

All good, thanks! I've applied those with a recipe, and that'll be in our PR checks going forward. :)