utils: Make behavior of ast_strsep* match strsep.

utils: Make behavior of ast_strsep* match strsep.

Given the scenario of passing an empty string to the
ast_strsep functions the functions would return NULL
instead of an empty string. This is counter to how
strsep itself works.

This change alters the behavior of the functions to
match that of strsep.

Fixes: #565
(cherry picked from commit 8ce69eda14c6564240a7f81640c1abad1cb76778)