func_presencestate: Return "not_set" when no data is set in AstDB

Return AST_PRESENCE_NOT_SET when CustomPresence AstDB key does not
exist, i.e. when a new CustomPresence is added in the dialplan.

ASTERISK-25400 #close
Reported by: Andrew Nagy

Change-Id: I6fb17b16591b5a55fbffe96f3994ec26b1b1723a

funcs/func_presencestate.c

@@ -255,7 +255,9 @@ static enum ast_presence_state custom_presence_callback(const char *data, char *
 	char *_message;
 	char *_subtype;
 
-	ast_db_get(astdb_family, data, buf, sizeof(buf));
+	if (ast_db_get(astdb_family, data, buf, sizeof(buf))) {
+		return AST_PRESENCE_NOT_SET;
+	}
 
 	if (parse_data(buf, &state, &_subtype, &_message, &_options)) {
 		return AST_PRESENCE_INVALID;