allow add_particles to be called with empty numpy arrays i.e. injec…

…ting zero particles (ECP-WarpX#2808)

Python/pywarpx/_libwarpx.py

@@ -591,11 +591,6 @@ def add_particles(self, species_name, x=None, y=None, z=None, ux=None, uy=None,
         for key, val in kwargs.items():
             assert np.size(val)==1 or len(val)==maxlen, f"Length of {key} doesn't match len of others"
 
-        # --- If the length of the input is zero, then quietly return
-        # --- This is not an error - it just means that no particles are to be injected.
-        if maxlen == 0:
-            return
-
         # --- Broadcast scalars into appropriate length arrays
         # --- If the parameter was not supplied, use the default value
         if lenx == 1: