Fix corner plotting when label is None

I was getting a bunch of errors like when parameters had label=None. This will fix those errors and make sure that the parameter key is used as the label.

File "/Users/colm/modules/bilby/bilby/core/result.py", line 1084, in plot_corner
    kwargs["labels"] = sanity_check_labels(kwargs["labels"])
  File "/Users/colm/modules/bilby/bilby/core/result.py", line 1902, in sanity_check_labels
    if "_" in lab and "$" not in lab:
TypeError: argument of type 'NoneType' is not iterable