Allow Param to support a default value of None

[jedcunningham]

Allow Param to support a default value of None.

This also stores the default value in value instead of default, since we later will update the resolved value.

closes: #18909

[jedcunningham]

I was running into differing id's with a naive object() sentinel, so I went with this class and used isinstance for comparison. Side benefit, allows for a better str/repr.

[uranusjr]

I think we now have like five different sentinel class implementations in the code base. I have one for DAG(schedule_interval). Probably should unify them somewhat into airflow.utils.types at some point.

Further reading for those interested: PEP 661: Sentinel Values

[kaxil]

On higher level it makes sense but would defer to @msumit

[msumit]

On higher level it makes sense but would defer to @msumit

lgtm

[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.