[BUG] conflict between import set_fields configuration and convert plugin

When the convert plugin is set to "auto: yes" and the import configuration has set_fields activated, the program will report error:
```
Traceback (most recent call last):
File "/usr/bin/beet", line 33, in
sys.exit(load_entry_point('beets==1.6.0', 'console_scripts', 'beet')())
File "/usr/lib/python3.10/site-packages/beets/ui/__init__.py", line 1285, in main
_raw_main(args)
File "/usr/lib/python3.10/site-packages/beets/ui/__init__.py", line 1272, in _raw_main
subcommand.func(lib, suboptions, subargs)
File "/usr/lib/python3.10/site-packages/beets/ui/commands.py", line 973, in import_func
import_files(lib, paths, query)
File "/usr/lib/python3.10/site-packages/beets/ui/commands.py", line 943, in import_files
session.run()
File "/usr/lib/python3.10/site-packages/beets/importer.py", line 340, in run
pl.run_parallel(QUEUE_SIZE)
File "/usr/lib/python3.10/site-packages/beets/util/pipeline.py", line 446, in run_parallel
raise exc_info[1].with_traceback(exc_info[2])
File "/usr/lib/python3.10/site-packages/beets/util/pipeline.py", line 311, in run
out = self.coro.send(msg)
File "/usr/lib/python3.10/site-packages/beets/util/pipeline.py", line 193, in coro
func(*(args + (task,)))
File "/usr/lib/python3.10/site-packages/beets/importer.py", line 1500, in import_asis
apply_choice(session, task)
File "/usr/lib/python3.10/site-packages/beets/importer.py", line 1523, in apply_choice
task.set_fields(session.lib)
File "/usr/lib/python3.10/site-packages/beets/importer.py", line 593, in set_fields
self.album.store()
File "/usr/lib/python3.10/site-packages/beets/library.py", line 1335, in store
super().store(fields)
File "/usr/lib/python3.10/site-packages/beets/library.py", line 344, in store
super().store(fields)
File "/usr/lib/python3.10/site-packages/beets/dbcore/db.py", line 534, in store
tx.mutate(
File "/usr/lib/python3.10/site-packages/beets/dbcore/db.py", line 866, in mutate
cursor = self.db._connection().execute(statement, subvals)
sqlite3.InterfaceError: Error binding parameter 2 - probably unsupported type.
```

Config file that generate this error:
```
convert:
auto: yes
quiet: yes
threads: 5
hardlink: true
tmpdir: /home/media/tmp
dest: /home/media/music/newage
album_art_maxwidth: 3000
no_convert:
format: opus
format: opus
formats:
opus:
command: ffmpeg -i $source -y -vn -acodec libopus -ab 128k $dest
extension: opus

import:
write: yes
copy: no
move: yes
hardlink: yes
resume: ask
incremental: no
incremental_skip_later: no
autotag: no
default_action: apply
quiet: yes
quiet_fallback: asis
none_rec_action: asis
timid: no
log: /home/media/config/beets/newage/beet.log
bell: yes
from_scratch: yes
set_fields:
albumtypes: {}
comments: {}
genre: 'New Age'
```