missing protection against empty runs

Python 3.6.4+ (default, Feb 12 2018, 08:25:03)
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IPython 6.2.1 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import spot

In [2]: a = spot.translate('1');

In [3]: r = spot.twa_run(a)

In [4]: r
Out[4]: 
Prefix:
Cycle:

In [5]: r.as_twa()
zsh: segmentation fault  ipython