Allow datasets without qualities to be downloaded.

[PGijsbers]

Allow datasets without qualities to be downloaded (because of the use of the cache function, subsequent get_dataset calls will always check if qualities are now available on the server). I am not aware of any dataset which will have no qualities indefinitely, so I don't know how to add a unit test or if we should.

[codecov-io]

Codecov Report

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The diff coverage is 50%.

@@            Coverage Diff             @@
##             develop     #847   +/-   ##
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  Coverage           ?   88.44%           
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  Files              ?       37           
  Lines              ?     4277           
  Branches           ?        0           
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  Hits               ?     3783           
  Misses             ?      494           
  Partials           ?        0

Impacted Files	Coverage Δ
openml/datasets/functions.py	94.06% <50%> (ø)

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