hval: to_filename: return `undef' instead of empty string

hval: to_filename: return `undef' instead of empty string

Returning an empty string for a filename makes no sense,
so instead return `undef' so the caller can setup a fallback
using the "//" operator.

This fixes uninitialized variable warnings because split()
on an empty string returns `undef', which caused to_filename
to warn on s// and tr// ops.