Problem description: Empty delimiter runtime error is observed when switching to "Test" tab in Regular expression configuration form, if an empty expression with expression type = "Character string not included" is configured in "Expressions" section.

Steps to reproduce:

1. Navigate to Administration => General => Regular expressions and press "New regular expression" button
2. In field "Expressions" set Expression type = "Character string included" and make sure field "Expression" is empty
3. Switch to "Test" tab

Result: The following runtime error is observed:

mb_strpos(): Empty delimiter [zabbix.php:21 -> require_once() -> ZBase->run() -> ZBase->processRequest() -> CController->run() -> CControllerRegExTest->doAction() -> CGlobalRegexp::matchExpression() -> CGlobalRegexp::_matchString() -> mb_strpos() -> CConfigFile->{closure}() in include/classes/regexp/CGlobalRegexp.php:207]

Expected: no errors should be triggered.