Misc cleanup.

core/field_dropdown.js

@@ -265,6 +265,9 @@ Blockly.FieldDropdown.prototype.getValue = function() {
  * @param {string} newValue New value to set.
  */
 Blockly.FieldDropdown.prototype.setValue = function(newValue) {
+  if (newValue === null || newValue === this.value_) {
+    return;  // No change if null.
+  }
   this.value_ = newValue;
   // Look up and display the human-readable text.
   var options = this.getOptions_();