MLLM train on empty corpus fails with error

[osma]

When trying to train MLLM on an empty corpus it fails with this:

  File "/data/Annif/annif/lexical/mllm.py", line 184, in prepare_train
    for candidates in train_x]), np.array(train_y))
  File "<__array_function__ internals>", line 6, in vstack
  File "/data/Annif/venv/lib/python3.6/site-packages/numpy/core/shape_base.py", line 283, in vstack
    return _nx.concatenate(arrs, 0)
  File "<__array_function__ internals>", line 6, in concatenate
ValueError: need at least one array to concatenate

It would be better to show an informative error, similar to the "cannot evaluate empty corpus" message.