Fix filter validation with not null filter

[stwiname]

Description

With a recent validation improvement to check filters exist in ABIs, there was no check to allow the !null filter. This adds it in.

Also defines a const to a void magic strings for '!null'

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have tested locally
• I have performed a self review of my changes
• Updated any relevant documentation
• Linked to any relevant issues
• I have added tests relevant to my changes
• Any dependent changes have been merged and published in downstream modules
• My code is up to date with the base branch
• I have updated relevant changelogs. We suggest using chan

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🔴	Statements	53.24%	3332/6258
🟡	Branches	68.39%	411/601
🔴	Functions	52.3%	148/283
🔴	Lines	53.24%	3332/6258

Test suite run success

87 tests passing in 14 suites.

Report generated by 🧪jest coverage report action from 9737ce8