Improve expressions to completely support `NULL` constants

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
Since DF has a real `NULL` support from #2364 , based on this pr, I'd like to file a ticket to track this trivial literal `NULL` supports for DF expressions.
- [x] #2364
- [x] #2478
- [x] #2482
- [x] #2609
- [ ] #2626
- [ ] Date, Timestemp comparisons with literal `NULL`
- [ ] Between expression works with literal `NULL`
- [ ] Regex expressions work with literal `NULL`

**Additional context**
This issue is based on #1184, thanks @alamb

No files or tests are named in the issue. Start by reviewing the linked NULL-support issues and the remaining checklist items, then locate the expression tests covering date and timestamp comparisons, BETWEEN, and regex expressions. Done means each listed expression handles a literal NULL consistently with existing NULL support.

Written by the indexing model from the issue text.