Initialize empty Matplotlib pane

[ahuang11]

Closes #6121

import matplotlib.pyplot as plt
import panel as pn
pn.extension()

mpl = pn.pane.Matplotlib()
mpl

fig, ax = plt.subplots()
ax.plot([1, 2, 3])
mpl.object = fig

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6a4d7f3) 84.67% compared to head (d3b64e7) 84.67%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6128   +/-   ##
=======================================
  Coverage   84.67%   84.67%           
=======================================
  Files         296      296           
  Lines       44096    44107   +11     
=======================================
+ Hits        37338    37348   +10     
- Misses       6758     6759    +1     

Flag	Coverage Δ
ui-tests	40.67% <18.18%> (-0.02%)	⬇️
unitexamples-tests	72.68% <100.00%> (+<0.01%)	⬆️

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