BUG: empty patches are not handled properly

[ctrltz]

Empty patches lead to an error when combining sources into stc, and the error message is not very descriptive.

 40%|███▉      | 99/250 [2:56:36<4:33:43, 108.76s/it]/data/p_02490/Scripts/meegsim/src/meegsim/sources.py:245: RuntimeWarning: divide by zero encountered in scalar divide
  scaling_factor = 1 / np.sqrt(len(self.vertno))

 40%|███▉      | 99/250 [2:56:47<4:29:39, 107.15s/it]
Traceback (most recent call last):
  File "/data/p_02490/Scripts/roiextract_simulations/scripts/entry_points/simulate.py", line 280, in <module>
    simulate(
  File "/data/p_02490/Scripts/roiextract_simulations/scripts/entry_points/simulate.py", line 205, in simulate
    sc = sim.simulate(
         ^^^^^^^^^^^^^
  File "/data/p_02490/Scripts/meegsim/src/meegsim/simulate.py", line 446, in simulate
    sources, noise_sources = _simulate(
                             ^^^^^^^^^^
  File "/data/p_02490/Scripts/meegsim/src/meegsim/simulate.py", line 513, in _simulate
    _adjust_snr_global(
  File "/data/p_02490/Scripts/meegsim/src/meegsim/snr.py", line 163, in _adjust_snr_global
    stc_signal = _combine_sources_into_stc(sources.values(), src, tstep)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/data/p_02490/Scripts/meegsim/src/meegsim/sources.py", line 370, in _combine_sources_into_stc
    vertices_stacked = np.vstack(vertices)
                       ^^^^^^^^^^^^^^^^^^^
  File "/data/u_kapralov_software/miniconda3/envs/roiextract_simulations/lib/python3.12/site-packages/numpy/core/shape_base.py", line 289, in vstack
    return _nx.concatenate(arrs, 0, dtype=dtype, casting=casting)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: all the input array dimensions except for the concatenation axis must match exactly, but along dimension 1, the array at index 0 has size 2 and the array at index 52 has size 0

[ctrltz]

To solve this, we need to check the locations and waveforms for the second time when generating sources. This problem won't arise if we just drop support for callable locations and waveforms.