Backport isoformat None guard

[uranusjr]

This was fixed in main in #17732, but the commit was not backported to 2.1 since the PR also contains a new feature and does not otherwise make sense on 2.1.

Fix #18286 #17814.

Not sure how the policy on 2.1 is right now.

[potiuk]

I do not think we plan another 2.1. release, but this one looks like easy to trigger and is pretty annoying, so maybe RC3 makes sense with that one @kaxil ?

[kaxil]

Does not look like this was a regression in 2.1.3 as 2.1.2 has the same. I say we wait for 2.2.0 - https://github.com/apache/airflow/blob/2.1.2/airflow/www/views.py#L851-L859 for this one and get 2.1.4 out asap as it fixes some core issues

[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest main or amend the last commit of the PR, and push it with --force-with-lease.