Document and test how ImmutableMap.toImmutableMap behaves when the …

…`mergeFunction` returns `null`.

(The test is Google-internal for now because we're in the process of reshuffling our `Collector` tests to make them run under Android as part of #6567. Also, we skip the test under GWT+J2CL because of a bug in their implementation of `Collectors.toMap`.)

This addresses the main part of #6824, but I'm keeping the bug open as a prompt to recognize our nullness annotations in the future.

RELNOTES=n/a
PiperOrigin-RevId: 580635720

guava/src/com/google/common/collect/ImmutableMap.java

@@ -96,8 +96,11 @@ public abstract class ImmutableMap<K, V> implements Map<K, V>, Serializable {
    * and values are the result of applying the provided mapping functions to the input elements.
    *
    * <p>If the mapped keys contain duplicates (according to {@link Object#equals(Object)}), the
-   * values are merged using the specified merging function. Entries will appear in the encounter
-   * order of the first occurrence of the key.
+   * values are merged using the specified merging function. If the merging function returns {@code
+   * null}, then the collector removes the value that has been computed for the key thus far (though
+   * future occurrences of the key would reinsert it).
+   *
+   * <p>Entries will appear in the encounter order of the first occurrence of the key.
    *
    * @since 21.0
    */