Improve nullness of types in some APIs related to Map merging, and fix Collectors.toMap null-handling.

[copybara-service]

Improve nullness of types in some APIs related to Map merging, and fix Collectors.toMap null-handling.

• Restrict Collections.toMap value-type arguments to non-nullable types.
  • ...in J2KT, following what we'd found in JSpecify research
• Fix Collections.toMap to remove the key in question when mergeFunction returns null.
  • ...in J2KT
  • ...in J2CL
• Use @NonNull / & Any in a few places in Map.merge and Map.computeIfPresent.
  • ...in J2KT
  • ...in Guava Map implementations, even though we don't yet include @NonNull annotations in the JDK APIs that we build Guava against. (See post-submit discussion on cl/559605577. But I've taken the shortcut of not waiting for the JDK APIs.)
• Use @Nullable (to match the existing Kotlin ? types) in the return types of Map.computeIfPresent and Map.compute.
  • ...in J2KT
• Test a bunch of this.

Note that the test for mergeFunction has to work around an overly restricted toMap signature that J2KT inherited from JSpecify. As discussed in a code comment there, this is fundamentally the same issue as we have in Guava with ImmutableMap.toImmutableMap, which is discussed as part of #6824.

RELNOTES=n/a