[gdb/testsuite] Fix Term::_csi_m with no args

[gdb/testsuite] Fix Term::_csi_m with no args

When calling Term::_csi_m with no args, default behaviour needs to be applied,
which is equivalent to "Term::_csi_m 0" [1].

However, while "Term::_csi_m 0" works, as well as 'Term::_csi_m ""', calling
Term::_csi_m with no args has no effect.

Fix this by implementing the default behaviour in Term::_csi_m.

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
[1] https://vt100.net/docs/vt510-rm/SGR.html