Suppress confusing uninit warning from -T _

Suppress confusing uninit warning from -T _

-T _ uses the file name saved by a preceding stat.  If there was no
preceding stat, the internal sv used to store the file name is unde-
fined, so SvPV producing an uninitialized warning.  Normally a failed
-T will just return undefined and set $!.  Normally stat on a filehan-
dle will set the internal stat file name to "".

This commit sets the internal file name to "" initially on startup,
instead of creating an undefined scalar.