Optional argument in `is*` functions about tempo

[WilcoSp]

If it would be implemented I do think it would be best to use typescript's function overload feature to provide more jsdoc, but also prevent unexpected behaviour from being created (like isAfter(null,null))

Also it's maybe best to first discuss together with Justin and other contributors which function categories and/or independent functions should allow for null/undefined arguments as this could have a big impact on Tempo

[WilcoSp]

this comes from the date function which converts an undefined value to a new Date object with the current time.

for at least the is* and diff* functions it might be a nice idea to make some argument(s) optional to use the current time

[Serator]

This would also work well with add* functions and such, but I don't see a way to do it without breaking the current design. Unless you use an explicit null as a keyword (addDay(null, 5)) or something like that, but it doesn't look like much. In any case, I would like a consistent solution.