[BUGFIX] Avoid undefined array key when determining strategy

[brotkrueml]

Resolves: #24

[PhilippKuhlmay]

Hi @brotkrueml
I get the same Error inside /page_speed_insights/Classes/FormEngine/Elements/History.php:48 when trying to edit the pagesettings of a page.
After changing this line into $this->strategyToShow = ($GLOBALS['TYPO3_CONF_VARS']['EXTCONF']['page_speed_insights']['strategyToShow'] ?? '') ?: 'mobile'; the error disappeared. So this line should be changed too. Thanks.