make sure repr workes with blank / fresh datasets

[amueller]

running create_dataset in jupyter was tricky because the repr failed afterwards.

[codecov-io]

Codecov Report

Merging #820 into develop will increase coverage by 0.33%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #820      +/-   ##
===========================================
+ Coverage    87.69%   88.02%   +0.33%     
===========================================
  Files           36       36              
  Lines         4234     4234              
===========================================
+ Hits          3713     3727      +14     
+ Misses         521      507      -14

Impacted Files	Coverage Δ
openml/datasets/dataset.py	87.21% <100%> (+3.69%)	⬆️
openml/_api_calls.py	84.61% <0%> (+1.28%)	⬆️

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