[5.x] filter_empty modifier

[marcorieser]

It adds a filter modifier that removes null values from an array. It accepts a param on whether to keep or discard the array's keys (default: true/keep).

[jasonvarga]

Competing with #10961 @godismyjudge95 🥊

[jasonvarga]

Calling this filter_empty to keep filter available if we ever want to support the full scope of filter.

Also, removed the option to remove keys. If you want to do that you can use the values modifier already.

[daun]

This probably closed statamic/ideas#1222.

[duncanmcclean]

This probably closed statamic/ideas#1222.

Thanks, closed!