Handle log colormapper zero lower bound

[philippjfr]

Ensures compatibility with changes to log colormapping and change to tuple palettes in bokeh 2.0.x

Also handles None in clim.

Fixes #4361
Fixes #4376

[jlstevens]

Looks good to me! I think with an augmented warning message, this can be merged.

[jbednar]

Looks good! It works well in my tests, but notice that there is a change in behavior: zero pixels are now transparent, when previously they were at the bottom of the color scale. I've pushed a change to nytaxi_hover.py to fix the display in that case, but there are probably other cases in the HoloViews codebase, datashader codebase, and examples.pyviz.org that will need a similar fix. Also note that the behavior for zero pixels now differs between linear and log colormapping, as 0 is only transparent for log. That will be confusing for Datashader users, but seems unavoidable.

[github-actions]

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