Bugfix: indexed row major serialization fails with None cell values

[yifanmai]

Previously, the indexed row major table serialization would fail if any cell values in the row were None with a stack trace like:

  File "/path/to/unitxt/struct_data_operators.py", line 95, in serialize
    return self.serialize_table(value)
  File "/path/to/unitxt/src/unitxt/struct_data_operators.py", line 133, in serialize_table
    serialized_tbl_str += self.process_row(row, row_index=i) + " "
  File "/path/to/unitxt/src/unitxt/struct_data_operators.py", line 149, in process_row
    serialized_row_str += " | ".join(row_cell_values)
TypeError: sequence item 2: expected str instance, NoneType found

This replaces the None values with the string "None", which is consistent with the other table serializers. An alternative would be to serialize them to the empty string "" instead, but this would be less consistent with the other table serializers.