Add null-type to several DOM-functions

[tvdijen]

These functions accept null for their namespace-param to use the default namespace.

[orklah]

Thanks for this!

You must have corresponding changes in Callmap_historical too (or possibly a delta file if those signatures changed in time). The full explanation is here: https://psalm.dev/docs/contributing/editing_callmaps/

[tvdijen]

Ah great, that was the info I was looking for!
These signatures haven't changed, so I've fixed the historical CallMap as well!

[orklah]

Thanks!