setenv(): Delete 0-length environment variables

Previously setenv() would only delete an environment variable if it
was passed a NULL string pointer as a value.  It should also delete an
environment variable when it encounters a valid string pointer of
0-length.

This change/fix is generally useful and is necessary for the upcoming
"editenv" command.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>