Fix multisearch array handling

[jessarcher]

While reviewing the issue raised in #130, I found some additional related issues with how the matches are determined after the user has selected something and then cleared the search field.

There were two main issues:

1. Array destructuring doesn't preserve numeric keys even if they are strings and/or non-sequential, causing issues in some scenarios with search results that use numeric ID keys where the keys should be returned instead of the values. In this scenario, the + operator is preferable, but it's not safe to use with lists.
2. array_is_list will return true on an empty array, so it's not safe to determine whether the selected values came from a list when the current $matches is empty. My solution is to capture whether $matches is a list the first time it is populated and use that for all future decisions.

This PR fixes these issues and also fixes the issue raised in #130 in a way that addresses the concerns I raised at #130 (comment).

Thanks to @macocci7 for finding the original issue and helping to solve it!