avutil/avstring: support input path as a null pointer or empty string

avutil/avstring: support input path as a null pointer or empty string

Linux and OSX systems support basename and dirname via <libgen.h>, I plan to
make the wrapper interface conform to the standard interface first.
If it is feasible, I will continue to modify it to call the system interface
if there is already a system call interface.

You can get more description about the system interface by below command:
 "man 3 basename"

Reviewed-by: Marton Balint <cus@passwd.hu>
Reviewed-by: Tomas Härdin <tjoppen@acc.umu.se>
Reviewed-by: Steven Liu <lq@chinaffmpeg.org>
Signed-off-by: Limin Wang <lance.lmwang@gmail.com>