gdb/mi: int to bool conversion in mi_execute_cli_command

gdb/mi: int to bool conversion in mi_execute_cli_command

Change an argument of mi_execute_cli_command from int to bool.  Update
the callers to take this into account.  Within mi_execute_cli_command,
update a comparison of a pointer to 0 with a comparison to nullptr,
and add an assert, if we are not using the argument string then the
string should be nullptr.  Also removed a cryptic 'gdb_????' comment,
which isn't really helpful.

There should be no user visible changes after this commit.