Fix hv.Empty not working in AdjointLayout plot

[Hoxbro]

Fixes #5559

Small regression, which made the following code error.

import holoviews as hv
import numpy as np

hv.extension("bokeh")

scatter = hv.Scatter(np.random.rand(100, 2))
scatter << hv.Empty() << scatter.hist(adjoin=False)

[codecov-commenter]

Codecov Report

Merging #5584 (8d426a4) into main (c306af3) will decrease coverage by 3.96%.
The diff coverage is 70.00%.

@@            Coverage Diff             @@
##             main    #5584      +/-   ##
==========================================
- Coverage   88.22%   84.26%   -3.97%     
==========================================
  Files         302      302              
  Lines       62185    62183       -2     
==========================================
- Hits        54864    52398    -2466     
- Misses       7321     9785    +2464     

Impacted Files	Coverage Δ
holoviews/ipython/display_hooks.py	59.70% <0.00%> (-0.61%)	⬇️
holoviews/core/layout.py	75.60% <100.00%> (ø)
holoviews/plotting/plot.py	93.25% <100.00%> (ø)
holoviews/tests/plotting/bokeh/test_layoutplot.py	100.00% <100.00%> (ø)
holoviews/tests/operation/test_datashader.py	1.75% <0.00%> (-95.87%)	⬇️
holoviews/operation/datashader.py	0.70% <0.00%> (-83.29%)	⬇️
holoviews/core/data/spatialpandas.py	14.70% <0.00%> (-72.61%)	⬇️
holoviews/tests/core/data/test_spatialpandas.py	34.13% <0.00%> (-64.08%)	⬇️
holoviews/core/data/spatialpandas_dask.py	50.79% <0.00%> (-31.75%)	⬇️
holoviews/tests/core/test_decollation.py	78.12% <0.00%> (-21.88%)	⬇️
... and 33 more

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[jlstevens]

Looks good to me!

Happy to see this merged once there are some unit tests. Docs updates can be in a separate PR.