Fix pickling state

[ahuang11]

Closes #6244

Not sure why it previously sets to an empty dict, but this makes pickling restore to its original state.

import pickle
import holoviews as hv
hv.extension("bokeh")

curve = hv.Curve([0, 1, 2], kdims=["XAXIS"])
pickle.loads(pickle.dumps(curve))

First introduced:
https://github.com/holoviz/holoviews/blame/849cd07779be75ff4e4366b3f3f13252afaa43d1/holoviews/core/dimension.py#L764

Prior to that (10 years ago xD):
https://github.com/holoviz/holoviews/blame/7c73cefe76c55d10323edcfcd14874cfdbcfc7a9/holoviews/core/dimension.py#L906

I think it might have been an oversight since AttrTree uses d
https://github.com/holoviz/holoviews/blob/main/holoviews/core/dimension.py#L1353

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.53%. Comparing base (dd0e4d5) to head (f47ffea).
Report is 1 commits behind head on main.

Files	Patch %	Lines
holoviews/core/dimension.py	88.88%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6245   +/-   ##
=======================================
  Coverage   88.53%   88.53%           
=======================================
  Files         323      323           
  Lines       67797    67797           
=======================================
+ Hits        60021    60024    +3     
+ Misses       7776     7773    -3     

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