Cannot reshape array of size 0 into shape (0,newaxis)
### Description
The error `cannot reshape array of size 0 into shape (0,newaxis)` shows up when trying to execute the following config on cartpole.
```
name | predicates | determinize | impurity | safe-pruning
----------------+-------------------+---------------+------------+----------------
another-config | ['linear-logreg'] | minnorm | entropy | False
```
On investigation, it turns out that the `check_done(dataset.x, y)` on line 120 of `decision_tree.py` is being called with both parameters being the empty list.
### Stacktrace
```
Traceback (most recent call last):
File "./dtcontrol/dtcontrol/benchmark_suite.py", line 95, in benchmark
cell, computed = self.compute_cell(ds, classifier)
File "./dtcontrol/dtcontrol/benchmark_suite.py", line 132, in compute_cell
cell = self.train_and_get_cell(dataset, classifier)
File "./dtcontrol/dtcontrol/benchmark_suite.py", line 141, in train_and_get_cell
classifier, success, run_time = call_with_timeout(classifier, 'fit', dataset, timeout=self.timeout)
File "./dtcontrol/dtcontrol/timeout.py", line 51, in call_with_timeout
getattr(obj, method)(*args)
File "./dtcontrol/dtcontrol/decision_tree/decision_tree.py", line 46, in fit
self.root.fit(dataset)
File "./dtcontrol/dtcontrol/decision_tree/decision_tree.py", line 133, in fit
node.fit(subset)
File "./dtcontrol/dtcontrol/decision_tree/decision_tree.py", line 133, in fit
node.fit(subset)
File "./dtcontrol/dtcontrol/decision_tree/decision_tree.py", line 133, in fit
node.fit(subset)
[Previous line repeated 20 more times]
File "./dtcontrol/dtcontrol/decision_tree/decision_tree.py", line 120, in fit
if self.check_done(dataset.x, y):
File "./dtcontrol/dtcontrol/decision_tree/decision_tree.py", line 147, in check_done
unique_data = np.unique(x, axis=0)
File "<__array_function__ internals>", line 6, in unique
File "./dtcontrol/venv/lib/python3.7/site-packages/numpy/lib/arraysetops.py", line 275, in unique
ar = ar.reshape(orig_shape[0], -1)
ValueError: cannot reshape array of size 0 into shape (0,newaxis)
```
### Reproducibility
First run `python cli.py preset --sample > ../user-config.yml` from inside the `dtcontrol` folder to create a `user-config.yml` file in the root. Then run `python cli.py --input ../examples/cartpole.scs --use-preset another-config --config ../user-config.yml`.
issue