gdb/testsuite: small cleanup in mi-break-qualified.exp

gdb/testsuite: small cleanup in mi-break-qualified.exp

It is not necessary to pass an empty string to mi_gdb_start, passing
the empty string is equivalent to passing no arguments, which is what
we do everywhere else (that we don't need to specify an actual
argument).

The only place we use 'mi_gdb_start ""' is in
gdb.mi/mi-break-qualified.exp, so in this commit I just replace that
with a call to 'mi_gdb_start' - just for consistency.

There should be no change in what is tested after this commit.