CMS 6: Remove backwards compatibility casting null to empty string in ArrayList::filterOrExclude()

[GuySartorelli]

In #10925 a new ArrayList::filterOrExclude() method was implemented which uses some null coalescing operators to turn null values (both in the objects contained in the list and the values to be passed to the SearchFilter) to empty strings. This is being done to preserve backwards compatibility with the old == comparison that arraylist filtering used to do.

From CMS 6, that backwards compatibility is no longer needed. It would be more appropriate to use strict comparisons for null values, to match up with how DataList and EagerLoadedList treat null value comparisons.

Acceptance criteria

• The null coalescing operators are removed from ArrayList::filterOrExclude() so that null values contained in the list and null values passed to the various filter methods are passed through to the searchfilters as null.
• This change is called out in the changelog

Notes

• That functionality was later changed in FIX Correctly implement backwards compatible null comparisons #10935 - we still need to pull out the legacy null comparisons but leaving this note here because it's not just a null coalescing operator anymore.