say.c: Prevent erroneous failures with 'say' family of functions.

A regression was introduced in ASTERISK~29531 that caused 'say'
functions to fail with file lists that would previously have
succeeded. This caused affected channels to hang up where previously
they would have continued.

We now explicitly check for the empty string to restore the previous
behavior.

ASTERISK-29859 #close

Change-Id: Ia2e5769868e2792313c2d7c07996efe009c6f8d5