valueFromAST: use nullish to simplify check (graphql#4573)

Author: yaacovCR

src/utilities/valueFromAST.ts

@@ -172,6 +172,6 @@ function isMissingVariable(
 ): boolean {
   return (
     valueNode.kind === Kind.VARIABLE &&
-    (variables == null || variables[valueNode.name.value] === undefined)
+    variables?.[valueNode.name.value] === undefined
   );
 }