Add null and equals filter types

[kkostov]

https://www.notion.so/telemetrydeck/TQL-Add-null-and-equals-filter-types-2b8947c6eda080009c14d4dad783f353?source=copy_link

Other changes:

• The CI script has been updated to use a Swift 6.2 image since the project no longer builds on 5.x for Linux
• I noticed the repo has a CLAUDE.md and this PR also updates it to match the current implementation (in case you're using LLM tools to help you out!)

[winsmith]

Great work! I just tested this on staging at https://agreeable-stone-050177a03-staging.westeurope.1.azurestaticapps.net and it works too! Feel free to merge!