Improve isNullOrUndefined return type (#9)

Author: bameyrick

src/isNullOrUndefined.ts

@@ -2,6 +2,6 @@
  * Detects whether a given value is null or undefined
  */
 // tslint:disable-next-line: no-any
-export function isNullOrUndefined(value: any): boolean {
+export function isNullOrUndefined(value: any): value is null | undefined {
   return value === null || value === undefined;
 }