Fix "-data-list-register-values" MI commands.

If register numbers are not provided then this commands is supposed
to return all the register values. But it was returning nothing in
that case.

More details about this command at
https://sourceware.org/gdb/onlinedocs/gdb/GDB_002fMI-Data-Manipulation.html#GDB_002fMI-Data-Manipulation

llvm-svn: 228304