Check case, when billing case does not present

Error with zero table for billing case. Example of error:

File "/data1/opsim_data/selfie_stick_testing/codes/pybill/pybill/billing.py", line 795, in processSh
    self.billCase2(shared_case2)
  File "/data1/opsim_data/selfie_stick_testing/codes/pybill/pybill/billing.py", line 337, in billCase2
    f = np.hstack(
  File "<__array_function__ internals>", line 180, in hstack
  File "/data/gal/selfie_stick_testingv2_env/lib/python3.10/site-packages/numpy/core/shape_base.py", line 345, in hstack
    return _nx.concatenate(arrs, 1)
  File "<__array_function__ internals>", line 180, in concatenate
ValueError: need at least one array to concatenat