Add empty progress bar

[hoxbro]

Fixes #2080

By setting the value to -1 the progress bar is empty.

import panel as pn

progress1 = pn.widgets.indicators.Progress(value=-1, active=False)
progress2 = pn.widgets.indicators.Progress(value=-1, active=True)
progress3 = pn.widgets.indicators.Progress(value=0, active=False)
progress4 = pn.widgets.indicators.Progress(value=0, active=True)
progress = pn.Column(progress1, progress2, progress3, progress4)
progress.servable()

progress.mp4

[codecov]

Codecov Report

Merging #2088 (f28d835) into master (238199f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2088   +/-   ##
=======================================
  Coverage   84.42%   84.42%           
=======================================
  Files         179      179           
  Lines       20776    20776           
=======================================
  Hits        17541    17541           
  Misses       3235     3235           

Impacted Files	Coverage Δ
panel/tests/widgets/test_misc.py	98.11% <100.00%> (ø)
panel/widgets/indicators.py	94.83% <100.00%> (ø)

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[philippjfr]

Interesting, I guess I don't quite understand why this works.

[hoxbro]

I think this could be a case of "It's not a bug, it's a feature".

[philippjfr]

Probably, the HTML spec can be...weird. Certainly would have expected it to treat 0 differently from null/unset.