Add Domain.empty, add warning to Domain.__bool__

[janezd]

Based on #2866 (1 commit). You may want to merge that request first.

Issue

#2866 fixes an issue with labels in MDS that occured since self.data.domain was False for a Domain with only meta attributes. This occurs since Domain.__len__ (correctly) returns the number of elements that Domain.__iter__ iterates across, however, Domain.__iter__ (incorrectly) ignores metas.

The issue will remain even after we fix Domain.__iter__: when checking whether a domain is true, we usually mean whether it's not None.

Description of changes

Domain has a new method, empty.

__bool__ warns that it's ambiguous and suggests using either empty or is None. The behaviour of __bool__ is unchanged - it returns True if the lenght of the domain is non-zero.

Includes

• Code changes
• Tests

[codecov-io]

Codecov Report

Merging #2867 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #2867   +/-   ##
======================================
  Coverage    82.1%   82.1%           
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  Files         328     329    +1     
  Lines       56210   56210           
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  Hits        46151   46151           
  Misses      10059   10059

[lanzagar]

Rebase please.