8221924: get(null) on single-entry unmodifiable Map returns null instead of throwing NPE

Reviewed-by: redestad, lancea

src/java.base/share/classes/java/util/ImmutableCollections.java

@@ -881,6 +881,11 @@ class ImmutableCollections {
             return Set.of(new KeyValueHolder<>(k0, v0));
         }
 
+        @Override
+        public V get(Object o) {
+            return o.equals(k0) ? v0 : null; // implicit nullcheck of o
+        }
+
         @Override
         public boolean containsKey(Object o) {
             return o.equals(k0); // implicit nullcheck of o