Add test for putIfAbsent method to verify replacement of null values.

This new test ensures that when a null value is present, the `putIfAbsent` method replaces it with a new value. The test checks both the return value and the updated state of the map.

Fixes #6217
Fixes #7987

RELNOTES=n/a
PiperOrigin-RevId: 807473633

Author: lmcrean

android/guava-testlib/src/com/google/common/collect/testing/testers/MapPutIfAbsentTester.java

@@ -126,4 +126,16 @@ public void testPut_nullValueSupported() {
         getMap().putIfAbsent(nullValueEntry.getKey(), nullValueEntry.getValue()));
     expectAdded(nullValueEntry);
   }
+
+  @MapFeature.Require({SUPPORTS_PUT, ALLOWS_NULL_VALUES})
+  @CollectionSize.Require(absent = ZERO)
+  public void testPutIfAbsent_replacesNullValue() {
+    initMapWithNullValue();
+
+    // putIfAbsent should replace the null value with the new value
+    assertNull(
+        "putIfAbsent(existingKeyWithNullValue, value) should return null",
+        getMap().putIfAbsent(getKeyForNullValue(), v3()));
+    assertEquals("Map should now contain the new value", v3(), getMap().get(getKeyForNullValue()));
+  }
 }

guava-testlib/src/com/google/common/collect/testing/testers/MapPutIfAbsentTester.java

@@ -125,4 +125,16 @@ public void testPut_nullValueSupported() {
         getMap().putIfAbsent(nullValueEntry.getKey(), nullValueEntry.getValue()));
     expectAdded(nullValueEntry);
   }
+
+  @MapFeature.Require({SUPPORTS_PUT, ALLOWS_NULL_VALUES})
+  @CollectionSize.Require(absent = ZERO)
+  public void testPutIfAbsent_replacesNullValue() {
+    initMapWithNullValue();
+
+    // putIfAbsent should replace the null value with the new value
+    assertNull(
+        "putIfAbsent(existingKeyWithNullValue, value) should return null",
+        getMap().putIfAbsent(getKeyForNullValue(), v3()));
+    assertEquals("Map should now contain the new value", v3(), getMap().get(getKeyForNullValue()));
+  }
 }