Add %checks to utility null/invalid checkers

Author: leebyron

src/jsutils/isInvalid.js

@@ -10,6 +10,6 @@
 /**
  * Returns true if a value is undefined, or NaN.
  */
-export default function isInvalid(value: mixed): boolean {
+export default function isInvalid(value: mixed): boolean %checks {
   return value === undefined || value !== value;
 }

src/jsutils/isNullish.js

@@ -10,6 +10,6 @@
 /**
  * Returns true if a value is null, undefined, or NaN.
  */
-export default function isNullish(value: mixed): boolean {
+export default function isNullish(value: mixed): boolean %checks {
   return value === null || value === undefined || value !== value;
 }