Don’t try to add `nil` strings to an array.

This can occur if one or more of the strings in `strarray` is invalid UTF8.

We’ve got a user seeing this when we’re pulling in their branch names. The on-disk `.git/refs/*` look ok, but I wonder if there’s something funky going on with the decomposed UTF16 filenames → assumed valid null-terminated UTF8 conversion in `@(cStr)`.

To wit, I think we should probably not be assuming valid null-terminated UTF8 here, but I’m not sure what we should assume instead.

I haven’t been able to synthesize a failing case (of filename → `nil` string literal) yet.