Add isNil and isUndefined

snippets/isNil.md

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+### isNil
+
+Returns `true` if the specified value is `null` or `undefined`, `false` otherwise.
+
+Use the strict equality operator to check if the value and of `val` are equal to `null` or `undefined`.
+
+```js
+const isNil = val => val === undefined || val === null;
+```
+
+```js
+isNil(null); // true
+isNil(undefined); // true
+```

snippets/isUndefined.md

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+### isUndefined
+
+Returns `true` if the specified value is `undefined`, `false` otherwise.
+
+Use the strict equality operator to check if the value and of `val` are equal to `undefined`.
+
+```js
+const isUndefined = val => val === undefined;
+```
+
+```js
+isUndefined(undefined); // true
+```