Wrong phpdoc @return value for getTitle()

When running tests against certain mobile platforms, the title is not available, and NULL is returned instead of a string. This mismatch between docs and reality has caused issues when using RemoteWebDriver in Codeception/PHPUnit trying to run assertions against the window title (`[PHPUnit_Framework_Exception] Argument #2 (No Value) of PHPUnit_Framework_Assert::assertNotContains() must be a array, traversable or string`)