Override ImmutableMap#getOrDefault for GWT/J2CL

This accomplishes two things:

Slight optimization as we don't need to check for key presence (ImmutableMap does not allow null values).
This allows the Checker Framework stubs to be applied.

RELNOTES=n/a
PiperOrigin-RevId: 861755893

Author: kevinoconnor7

guava-gwt/src-super/com/google/common/collect/super/com/google/common/collect/ImmutableMap.java

@@ -435,6 +435,17 @@ public boolean containsValue(@Nullable Object value) {
     return values().contains(value);
   }
 
+  @Override
+  public final @Nullable V getOrDefault(@Nullable Object key, @Nullable V defaultValue) {
+    V result = get(key);
+    // TODO(b/192579700): Use a ternary once it no longer confuses our nullness checker.
+    if (result != null) {
+      return result;
+    } else {
+      return defaultValue;
+    }
+  }
+
   private transient @Nullable ImmutableSet<Entry<K, V>> cachedEntrySet = null;
 
   @Override