GPL-984 BUG - Exceptions while processing UK Biocentre

[pjvv]

Describe the bug
Seen in slack message:

2021-04-17 20:06:08,521 crawler.main                                 :113 ERROR   expected string or bytes-like object
Traceback (most recent call last):
 File "/code/crawler/main.py", line 109, in run
   centre_instance.process_files(add_to_dart)
 File "/code/crawler/file_processing.py", line 170, in process_files
   centre_file.process_samples(add_to_dart)
 File "/code/crawler/file_processing.py", line 404, in process_samples
   docs_to_insert = self.process_csv()
 File "/code/crawler/file_processing.py", line 773, in process_csv
   documents = self.parse_and_format_file_rows(csvreader)
 File "/code/crawler/file_processing.py", line 1044, in parse_and_format_file_rows
   if parsed_row := self.parse_and_format_row(row, line_number, seen_rows):
 File "/code/crawler/file_processing.py", line 1080, in parse_and_format_row
   modified_row = self.filtered_row(row, line_number)
 File "/code/crawler/file_processing.py", line 936, in filtered_row
   seen_headers, modified_row = self.extract_channel_fields(seen_headers, row, modified_row)
 File "/code/crawler/file_processing.py", line 1001, in extract_channel_fields
   if pattern.match(csv_field):
TypeError: expected string or bytes-like object