BUG: Groupby median on timedelta column with NaT returns odd value (#…
…57926)

Handle NaT correctly in group_median_float64

- [x] closes #57926
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Thanks @JJLLWW  @WillAyd - why the backport? I didn't think this was a regression. Ah OK - I thought we were doing bugfixes on that branch now. Didn't realize it was limited to regressions. Can just keep this on 3.x then This might be just me, but I like to keep patch versions as small as possible since bugfixes can induce new bugs and would like patch upgrade to be as safe as possible for users. But in the past, I know others have been more inclined to backport more than I prefer - cc @phofl for any thoughts. Agree with @rhshadrach 

we backported fixes for new features that were introduced in the version before but generally not much else > we backported fixes for new features that were introduced in the version before but generally not much else

Ah, I think that's where I remember a disagreement - thanks. And yea, I've come around to doing bug fixes for new features.