Function isNotNull

isNotNull<T>(u: T | null): u is T
Type guard that checks if a value is not null.

Type Narrowing Behavior:
- Excludes null from the input type when true
- Preserves all other types including undefined
- Commonly used to filter out null values
Type Parameters
- T
  The type of the input value (which could be null)
Parameters
- u: T | null
  The value to check
Returns u is T
true if u is not null, false otherwise. When true, TypeScript excludes null from the type.
Example
```
const values: (number | null)[] = [null, 5];

const nonNullValues = values.filter(isNotNull);

assert.deepStrictEqual(nonNullValues, [5]);
```
- Defined in src/guard/is-type.mts:355