I came up with a hack that seems to solve my problem
The two bits of code are included in if statements that activate if they are Y or non-empty respectively. My solution was to extend those if statements to also check whether the variable in question is set, like so:
if (isset($arrSettings['ssba_custom_styles_enabled']) == '' || $arrSettings['ssba_custom_styles_enabled'] != 'Y') {
if (isset($arrSettings['ssba_additional_css']) != '' && $arrSettings['ssba_additional_css'] != '') {
It’s not particularly pretty, but it works.
