Fixing bug when executing a breaking pset

[erikvansebille]

Previously, running a pset.execute() on an empty set returned an Error

Traceback (most recent call last):
  File "/Users/erik/Codes/ParcelsCode/parcels/particleset.py", line 907, in execute
    min_rt = np.min(self.particledata.data['time_nextloop'])
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/erik/anaconda3/envs/parcels/lib/python3.12/site-packages/numpy/core/fromnumeric.py", line 2953, in min
    return _wrapreduction(a, np.minimum, 'min', axis, None, out,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/erik/anaconda3/envs/parcels/lib/python3.12/site-packages/numpy/core/fromnumeric.py", line 88, in _wrapreduction
    return ufunc.reduce(obj, axis, dtype, out, **passkwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: zero-size array to reduction operation minimum which has no identity

This PR fixes that Issue by gracefully returning from pset.execute when there are no particles to execute. This fixes #1629