Implement nonzero/bool methods to override len

[philippjfr]

Currently when testing any HoloViews object for "truthiness" it uses the __len__ method. In #974 we added the dask interface, which doesn't actually know it's length because it loads the data lazily. Therefore we should implement __nonzero__ (Python 2) and __bool__ (Python 3) methods, which take precedence over the length. Additionally we should confirm that all the code in HoloViews uses the implicit boolean truthiness to check for a zero length element rather than explicitly calling len.

[philippjfr]

Implemented in PR referenced above.

[github-actions]

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