Fix is_null_or_undefined() call in codegen to be snake_case.

After a bit of digging, I couldn't find when it was camelCase.  This
started getting generated when I added an overload in webgl.

components/script/dom/bindings/codegen/CodegenRust.py

@@ -398,7 +398,7 @@ def pickFirstSignature(condition, filterLambda):
                 return False
 
             # First check for null or undefined
-            pickFirstSignature("%s.isNullOrUndefined()" % distinguishingArg,
+            pickFirstSignature("%s.get().is_null_or_undefined()" % distinguishingArg,
                                lambda s: (s[1][distinguishingIndex].type.nullable() or
                                           s[1][distinguishingIndex].type.isDictionary()))