fixed the error for associate column within the material sheet. (#44)

Essentially we are checking if the column is empty within pandas and if it is, then we use None instead of NaN because JSON can handle None and not NaN

src/python/parse.py

@@ -105,7 +105,7 @@ def _get_cell_info(self, index, row, column):
  "type": nested_types[-1],
  "unique_key": nested_keys[-1].strip("*"),
  "key": self._clean_key(nested_keys[-1]),
- "value": row[column],
+ "value": row[column] if not pd.isna(row[column]) else None,
  "unit": column[2] if "Unnamed" not in column[2] else None,
  }