Shell: Update: Print error if no update file name is specified

stm-firmware/shell.c

@@ -655,6 +655,11 @@ shellmatta_retCode_t shell_cmd_update(const shellmatta_handle_t handle, const ch
 		}
 	}
 
+	if (!update_file || !strlen(update_file)) {
+		shellmatta_printf(handle, "Please specify a valid update file!\r\n");
+		return SHELLMATTA_ERROR;
+	}
+
 	updater_update_from_file(update_file);
 
 	return SHELLMATTA_OK;