Add back +1 element with explicit NULL

Fun fact: this is required by ANSII C:
- https://stackoverflow.com/a/39096006/2719194
- https://code.activestate.com/lists/python-list/704158

and we did not yet initialize it properly.

Python/pywarpx/_libwarpx.py

@@ -402,14 +402,20 @@ def get_nattr_species(self, species_name):
     def amrex_init(self, argv, mpi_comm=None):
         # --- Construct the ctype list of strings to pass in
         argc = len(argv)
-        self.argvC = (_LP_c_char * argc)()
+        # note: +1 since there is an extra char-string array element,
+        #       that ANSII C requires to be a simple NULL entry
+        #       https://stackoverflow.com/a/39096006/2719194
+        #       https://code.activestate.com/lists/python-list/704158
+        self.argvC = (_LP_c_char * (argc + 1))()
         print("amrex_init argv=", argv)
         print("amrex_init len(argv)=", len(argv))
         for i, arg in enumerate(argv):
             enc_arg = arg.encode('utf-8')
             print(enc_arg, len(enc_arg), repr(enc_arg), type(enc_arg))
             #self.argvC[i] = ctypes.create_string_buffer(enc_arg)
             self.argvC[i] = ctypes.c_char_p(enc_arg)
+        self.argvC[argc] = ctypes.c_char_p(b"\0")
+
         print("len(argvC)=", len(self.argvC))
         self.argvC = ctypes.cast(self.argvC, _LP_LP_c_char)
         print("argc=", argc)