Bug in flexmeasures show beliefs in case of empty sensors

[Flix6x]

The dataframe handed over to uniplot cannot contain empty columns. Otherwise, you get:

ValueError: zero-size array to reduction operation maximum which has no identity

We knew that, and were already throwing out columns with no sensor data. But we did it with a bad loop, that removes items from the list while looping over that same list. In case of a list where the non-empty sensors aren't all at the back of the list, we still end up trying to show empty sensor data.

[coveralls]

Pull Request Test Coverage Report for Build 3546346177

• 0 of 4 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 64.977%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
flexmeasures/cli/data_show.py	0	4	0.0%

Totals
Change from base Build 3544058178:	-0.02%
Covered Lines:	6586
Relevant Lines:	9516

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💛 - Coveralls