php 7.3 compact() calls with undefined variables causing error page

With php 7.3 changes to compact(), the warnings of missing variables are causing a warning that stops execution.

So far, I only found these problems in search.php and phpbb/search/fulltext_native.php.

I think more places should be tested and fixed before releasing version 3.3.

I tested this on the current master (commit: 88b4dbfc943077fc685344c566d292b4b07cb7af)