Add default impl to IConfigElement#getValidValuesDisplay

src/main/java/net/minecraftforge/fml/client/config/IConfigElement.java

@@ -176,7 +176,10 @@ public interface IConfigElement
      * [Property] Gets a String array of the versions of this property's valid values that will display in the config GUI.
      * This is generally used for String properties to allow the user to select a value from a list of valid values.
      */
-    String[] getValidValuesDisplay();
+    default String[] getValidValuesDisplay()
+    {
+    	return null;
+    }
 
     /**
      * [Property] Gets this property's minimum value.