made nullSafe wrapper of JsonAdapter optional

gson/src/main/java/com/google/gson/annotations/JsonAdapter.java

@@ -94,4 +94,7 @@ public @interface JsonAdapter {
   /** Either a {@link TypeAdapter} or {@link TypeAdapterFactory}. */
   Class<?> value();
 
+  /** false, to be able to handle {@code null} values within the adapter, default value is true. */
+  boolean nullSafe() default true;
+
 }

gson/src/main/java/com/google/gson/internal/bind/JsonAdapterAnnotationTypeAdapterFactory.java

@@ -73,7 +73,7 @@ public final class JsonAdapterAnnotationTypeAdapterFactory implements TypeAdapte
               + "JsonSerializer or JsonDeserializer reference.");
     }
 
-    if (typeAdapter != null) {
+    if (typeAdapter != null && annotation.nullSafe()) {
       typeAdapter = typeAdapter.nullSafe();
     }