Fix null pointer bug when -verifyroms is used without a driver spec (nw)

This undoes a small part of c6ef2746d4. Changing "*" to nullptr here looks like a fundamentally unsound hack, even though the validity checker now uses it as a convenient flag (see the similar change in 3ecdaafea6).

src/frontend/mame/clifront.cpp

@@ -1501,7 +1501,7 @@ void cli_frontend::execute_commands(const char *exename)
 		{
 			// parse any relevant INI files before proceeding
 			const char *sysname = m_options.system_name();
-			(this->*info_command.function)((sysname[0] == 0) ? nullptr : sysname);
+			(this->*info_command.function)((sysname[0] == 0) ? "*" : sysname);
 			return;
 		}
 	}