pybind: decode empty string in conf_parse_argv() correctly

pybind: decode empty string in conf_parse_argv() correctly

cretargs is a array of c_char_p, which means ctypes has already
converted it to python byte strings. decode_cstr() would misinterpret
the empty string as a NULL c_char_p(), and convert it to None by
accident, resulting in errors when running commands like
'ceph config-key put foo ""'.

Since this is the only place we use arrays of c_char_p, just decode
it directly in conf_parse_argv(). Tested with python 2 and 3.

Signed-off-by: Josh Durgin <jdurgin@redhat.com>