Typeguard against null[] - arrays

Question

I am working with some autogenerated types from graphQL-data and need a way to make a typeguard against null, undefined and null[] to narrow types.
So far the I got this:
export default function validData<T>(
condition: T
): asserts condition is Exclude<T, null | undefined | null[]> {
if (
condition === null ||
condition === undefined ||
(Array.isArray(condition) && typeof condition[0] === null)
) {
throw new Error(
`missing data: ${condition} returned null, null[] or undefined`
);
}
}
And then I simply use the function like this:
validData(menuDataToNarrow):
The function is working against null and undefined, but I can't seem to wrap my head around how to catch and throw errors if the data contains an empty array? The above code was my best (non-working) attempt so far.
The types are narrowed as expected when it comes to null and undefined, but I still get:
XXX is not assignable to type null[]
Any idea how to fix this?
Edit: I came up with this finished guard that works:
export default function validData<T>(
condition: T
): asserts condition is Exclude<T, null | undefined | null[]> {
if (
condition === null ||
condition === undefined ||
(Array.isArray(condition) && condition.every((x) => x === null))
) {
throw new Error(
`missing data: ${condition} returned null, null[] or undefined`
);
}
}

Answer 1

null[] Would be an array with nulls. You can check that with Array.prototype.every()
I think you are looking for 'an empty array' which is [] an array with length 0 or Array.isArray(data) && data.length === 0 as #VLAZ suggested in the comments
Edit based on your comment:
Be careful to correctly check for null and/or undefined in your code. I've tried defining an empty array but it does not seem trivial in TS