multi options field - account for querying by ':empty:', ':notempty:' values

[i-just]

Description

When querying the multi-options field (checkboxes or multi-select), account for querying by ':empty:' and ':notempty:' values.
When column type is JSON, the empty value can be stored as null, empty string, or empty array (`'[]``).

Steps to reproduce:

• create one of each: checkboxes, radio, multi-select, dropdown field; each with options “a”, “b”, “c”
• create a section with an entry type that has all those fields
• create the following entries in that section:
  • “test 1” => all fields are empty
  • “test 2” => fill out checkboxes and multi-select with options: “a”, “b”
  • “test 3” => fill out the radio and dropdown with the option “a”
  • “test 4” => fill out checkboxes and multi-select with options: “a”, “c” and radio and dropdown with the option “b”

Filter the element index table by each field (one at a time) and test that “is empty” and “is not empty” work as expected.

Set up a template that queries by multiple values where one of them is ':empty:' or ':notempty:'. For example:

<h3>multi select</h3>
{% set entries = craft.entries.section('oli').myMultiSelect(['b',':empty:']).orderBy('title').all() %}
{% for entry in entries %}
    <p>{{ entry.title }}</p>
{% endfor %}
<hr />

<h3>checkboxes</h3>
{% set entries = craft.entries.section('oli').myCheckboxes(['b', ':empty:']).orderBy('title').all() %}
{% for entry in entries %}
    <p>{{ entry.title }}</p>
{% endfor %}
<hr />

<h3>dropdown</h3>
{% set entries = craft.entries.section('oli').myDropdown(['b', ':empty:']).orderBy('title').all() %}
{% for entry in entries %}
    <p>{{ entry.title }}</p>
{% endfor %}
<hr />

<h3>radio</h3>
{% set entries = craft.entries.section('oli').myRadio(['b', ':empty:']).orderBy('title').all() %}
{% for entry in entries %}
    <p>{{ entry.title }}</p>
{% endfor %}

The result (with this PR):

• multi select => test 1, test 2, test 3
• checkboxes => test 1, test 2, test 3
• dropdown => test 1, test 2, test 4
• radio => test 1, test 2, test 4

The result without this PR:

• multi select => test 2
• checkboxes => test 2
• dropdown => test 1, test 4
• radio => test 1, test 2, test 4

Related issues

n/a; reported by @olivierbon

[i-just]

I had to put it back into draft as the second commit is causing other issues.