Fixing https://github.com/google/gson/issues/1592 (#1623)

adding null parameter check to name(), fixing the issue and being consistent with superclass behavior

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

@@ -130,6 +130,9 @@ public final class JsonTreeWriter extends JsonWriter {
   }
 
   @Override public JsonWriter name(String name) throws IOException {
+    if (name == null) {
+      throw new NullPointerException("name == null");
+    }
     if (stack.isEmpty() || pendingName != null) {
       throw new IllegalStateException();
     }