Add test for putIfAbsent to catch implementations that incorrectly ignore null values

[lmcrean]

Fixes #6217

Problem

Multiple map implementations (e.g., fastutils' Object2ObjectOpenHashMap) incorrectly pass Guava's testlib suite despite not properly implementing Map.putIfAbsent behavior with null values. The JavaDoc specifies that putIfAbsent should replace a null value, but the testlib doesn't verify this edge case, allowing non-compliant implementations to pass.

Solution

Add testPutIfAbsent_replacesNullValue() to verify that putIfAbsent correctly replaces existing null values with new values, returning null as specified in the Map interface documentation.

Changes

• Added new test method in MapPutIfAbsentTester.java that:
  • Sets an existing key's value to null
  • Calls putIfAbsent with a new value
  • Verifies it returns null (the old value)
  • Confirms the key now maps to the new value

Testing

# Compile the testlib
./mvnw compile -pl guava-testlib

# Run tests that exercise MapPutIfAbsentTester with null-supporting maps
./mvnw test -Dtest=OpenJdk6MapTests -pl guava-testlib
./mvnw test -Dtest=HashMapTest -pl guava-tests
./mvnw test -Dtest=LinkedHashMapTest -pl guava-tests

Breaking Changes

None. This only adds a new test case to catch non-compliant Map implementations.

[cpovirk]

Thanks, I'll make sure that this doesn't break any internal users of the test suites (or fix them if it does) and then get it merged.