openml.datasets.get_data() has unpredictable number of return values

MWE:
```
dataset = openml.datasets.get_dataset(40978)

res = dataset.get_data(target=dataset.default_target_attribute)
print(len(res))
dataset = openml.datasets.get_dataset(2)

res = dataset.get_data(target=dataset.default_target_attribute)
print(len(res))
```
This depends on whether a default target attribute has been specified. 