Add "OR IS NULL" option to views filter of type Group

Patch for D9/10

Changed to use Database::getConnection()->condition() instead of new Condition('OR') as per https://www.drupal.org/node/3159568 →

There are more issues around that try to find a way of dealing with NULL value fields, see #1919834: Field instance got no default value when created in field UI → and the https://www.drupal.org/project/field_defaults → module. That to me is the right solutions if you want to deal with NULL values for fields, not mess with hard to find settings in your Views. See ✨ Views UI should offer a 'Treat NULL values as FALSE' on boolean field filtering to expose the already existing code path for handling this Needs work and all its related issues for boolean fields trying to handle this.

But, if this is something you want to do in Views anyway, this sounds like a reasonable request since it would be hard to do something like this in contrib.

+++ b/core/modules/views/config/schema/views.data_types.schema.yml
@@ -756,6 +756,9 @@ views_filter:
+        include_null:
+          type: boolean
+          label: 'Include NULL values'

+++ b/core/modules/views/src/Plugin/views/filter/FilterPluginBase.php
@@ -161,6 +161,7 @@ protected function defineOptions() {
+        'include_null' => ['default' => FALSE],

@@ -843,6 +844,7 @@ protected function buildGroupOptions() {
+      'include_null' => FALSE,

This would require an upgrade path to set the default in existing Views.

+++ b/core/modules/views/config/schema/views.data_types.schema.yml
--- a/core/modules/views/src/Plugin/views/filter/FilterPluginBase.php
+++ b/core/modules/views/src/Plugin/views/filter/FilterPluginBase.php

+++ b/core/modules/views/src/Plugin/views/filter/FilterPluginBase.php
--- a/core/modules/views/src/Plugin/views/filter/StringFilter.php
+++ b/core/modules/views/src/Plugin/views/filter/StringFilter.php

The option is added to the FilterPluginBase but only used in StringFilter, so shouldn't we add it to StringFilter?

Also, this needs tests.

Original patch only applied to StringFilter. I expanded it so it also applies to NumericFilter.

This might be a valid reason to leave the option in FilterPluginBase (since it's being questioned in #4).
Upgrade path + test coverage is still TODO.