Check that needle is not empty before searching for it.

[donnchawp]

If the CDN setting "Exclude if substring" is blank a warning will be
generated when you look for it. This fixes that and uses stripos instead
of stristr as it's faster.
ref: https://wordpress.org/support/topic/another-empty-needle-issue/

[stodorovic]

Just an improvement, it's little faster (and easier) to use array_filter together with array_map. For example:
$arr_of_excludes = array_filter( array_map( 'trim', explode( ',', $ossdl_off_exclude ) ) ) ;

I'm thinking to move this code only on update settings. We should save array instead string, then it's possible to we use quicker way for loading - $ossdl_off_exclude = get_option('ossdl_off_exclude'). Main trouble is function which converts current options into new format. So, it makes sense if we convert more options on this way.