Add option to clear value of DatetimePicker

[hoxbro]

Fixes #3975

This PR will make it possible to clear the value of DatetimePicker.

Example:

import panel as pn
from datetime import datetime

pn.extension()

time_widget = pn.widgets.DatetimePicker()
btn = pn.widgets.Button(name="click")

@pn.depends(btn, watch=True)
def tmp(*event):
    if time_widget.value:
        time_widget.value = None
    else:
        time_widget.value = datetime.today()


pn.Column(btn, time_widget).servable()

screenrecord-2022-10-12_18.41.40.mp4

TODO:

• Write unit test for it
• Add support for other np.nan / pd.NaT

[codecov]

Codecov Report

Merging #3990 (840b44e) into master (a5bd963) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3990      +/-   ##
==========================================
- Coverage   83.74%   83.70%   -0.05%     
==========================================
  Files         223      223              
  Lines       32971    33080     +109     
==========================================
+ Hits        27611    27689      +78     
- Misses       5360     5391      +31     

Flag	Coverage Δ
ui-tests	37.55% <100.00%> (+0.59%)	⬆️
unitexamples-tests	74.76% <16.21%> (-0.22%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
panel/models/datetime_picker.py	100.00% <100.00%> (ø)
panel/tests/ui/widgets/test_input.py	99.47% <100.00%> (+0.05%)	⬆️
panel/tests/ui/io/test_convert.py	95.37% <0.00%> (-3.49%)	⬇️
panel/pane/image.py	79.53% <0.00%> (-0.66%)	⬇️
panel/io/state.py	69.76% <0.00%> (-0.50%)	⬇️
panel/io/pyodide.py	0.00% <0.00%> (ø)
panel/command/convert.py	31.42% <0.00%> (ø)
panel/tests/test_server.py	99.80% <0.00%> (+<0.01%)	⬆️
panel/widgets/tables.py	88.40% <0.00%> (+0.09%)	⬆️
panel/io/resources.py	86.80% <0.00%> (+0.09%)	⬆️
... and 5 more

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[philippjfr]

Looks good!