runtime: Do not error when writing zero bytes to a File

[cg-jl]

File::write might get an empty array, so fwrite() returning zero does not always mean that an error occurred. Better check that an error actually occurred before returning an Error(errno=0).

[BuggieBot]

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[awesomekling]

How about we check for 0 before even calling fwrite, and early return if so?

[cg-jl]

Seems like a better option, since that's the only other case where fwrite() can return zero without having an error.