[ticket/11437] add an extra condition to if statement

When search returns no results there is no need to go inside the if
statement. Since $result_count becomes zero, $start becomes negative
which leads to failed assertion.

PHPBB3-11437
[ticket/11437] add basic search test structure

PHPBB3-11437
[ticket/11437] Skip tests for unsupported search backends

PHPBB3-11437
[ticket/11437] complete search tests functionalities

PHPBB3-11437
[ticket/11437] Use assertions in test

PHPBB3-11437
[ticket/11437] Add more assetions in tests

PHPBB3-11437
[ticket/11437] Logout before searching
Keyword search in search functional tests should be
performed as guests rather than logged in as admin.

PHPBB3-11437