Performance fix to avoid reflection-based initialization of null key surrogate

Author: yasirmhd

gson/src/main/java/com/google/gson/Gson.java

@@ -109,7 +109,7 @@ public final class Gson {
   static final boolean DEFAULT_COMPLEX_MAP_KEYS = false;
   static final boolean DEFAULT_SPECIALIZE_FLOAT_VALUES = false;
 
-  private static final TypeToken<?> NULL_KEY_SURROGATE = new TypeToken<Object>() {};
+  private static final TypeToken<?> NULL_KEY_SURROGATE = TypeToken.get(Object.class);
   private static final String JSON_NON_EXECUTABLE_PREFIX = ")]}'\n";
 
   /**