Sign0 predict key_type needs to be set

In the new sign0 for the predict method if the parameter key_type is not set the method fails. The documentation says: key_type(str): Type of key. May be inchikey or smiles. If None specified, no filtering is applied (default=None). But it fails in the sorting.

2020-04-01 11:25:22,984 chemicalchecker.core.sign0.sign0 [DEBUG   ] Setting input type
2020-04-01 11:25:22,984 chemicalchecker.core.sign0.sign0 [DEBUG   ] Sorting
Traceback (most recent call last):
  File "/aloy/scratch/oguitart/tmp/jobs_sign0_pred_74z5ryxm/sign0_pred_script.py", line 19, in <module>
    sign_full.predict(**pars)
  File "/aloy/home/oguitart/projects/source/chemical_checker/package/chemicalchecker/core/sign0.py", line 354, in predict
    agg_method=self.agg_method)
  File "/aloy/home/oguitart/projects/source/chemical_checker/package/chemicalchecker/core/sign0.py", line 174, in get_data
    X, keys, keys_raw, features = self.sort(X, keys, keys_raw, features)
  File "/aloy/home/oguitart/projects/source/chemical_checker/package/chemicalchecker/core/sign0.py", line 243, in sort
    keys_raw = keys_raw[key_idxs]
TypeError: 'NoneType' object is not subscriptable