little bug of biotite.structure.dot_bracket_from_structure

[flyfyyfyy]

When the input of biotite.structure.dot_bracket_from_structure doesn't have any base pairs, biotite.structure.base_pairs will return an empty NumPy array array([], dtype=float64).
When np.sort(basepairs, axis=1) is applied to this empty array, it will raise an AxisError: axis 1 is out of bounds for array of dimension 1, causing the dot_bracket_from_structure function to fail.

Of course, that was just a minor issue. Your package is really powerful.

[t0mdavid-m]

Hi,
thanks a lot for the bug report! I will look into it over the next couple days.