Function isNull

isNull(u: unknown): u is null
Type guard that checks if a value is null.

Type Narrowing Behavior:
- Narrows the input type to null when true
- Useful for explicit null checks
Parameters
- u: unknown
  The value to check
Returns u is null
true if u is null, false otherwise. When true, TypeScript narrows the type to null.
Example
```
const values: readonly (number | null)[] = [null, 5] as const;

const nullValues = values.filter(isNull);

assert.deepStrictEqual(nullValues, [null]);
```
- Defined in src/guard/is-type.mts:329