Fixed UNDEFINED TIFF tag of length 0 being changed in roundtrip

[radarhere]

from PIL import Image
im = Image.open("Tests/images/hopper.tif")
im.tag_v2[45059] = b"test"
print(im.tag_v2[45059])  # (b'test',)

# You would think that self-assignment shouldn't change the value
im.tag_v2[45059] = im.tag_v2[45059]

# But it does
print(im.tag_v2[45059])  # ((b'test',),)

I would maintain that this should not be the case, so this PR fixes that.

What is special about this tag is that it is UNDEFINED with length 0.

Pillow/src/PIL/TiffTags.py

Line 192 in ab8955b

45059: ("ImageUIDList", UNDEFINED, 0), # UNDONE, check

The value of an UNDEFINED tag is always turned into a list in _setitem - meaning that if it is already a sequence, it gets wrapped again.

Pillow/src/PIL/TiffImagePlugin.py

Lines 566 to 569 in ab8955b

	if self.tagtype[tag] == TiffTags.UNDEFINED:
	values = [
	value.encode("ascii", "replace") if isinstance(value, str) else value
	]

And because the length is 0, at the end of _setitem it isn't unwrapped.

Pillow/src/PIL/TiffImagePlugin.py

Lines 584 to 608 in ab8955b

	if not is_ifd and (
	(info.length == 1)
	or self.tagtype[tag] == TiffTags.BYTE
	or (info.length is None and len(values) == 1 and not legacy_api)
	):
	# Don't mess with the legacy api, since it's frozen.
	if legacy_api and self.tagtype[tag] in [
	TiffTags.RATIONAL,
	TiffTags.SIGNED_RATIONAL,
	]: # rationals
	values = (values,)
	try:
	(dest[tag],) = values
	except ValueError:
	# We've got a builtin tag with 1 expected entry
	warnings.warn(
	f"Metadata Warning, tag {tag} had too many entries: "
	f"{len(values)}, expected 1"
	)
	dest[tag] = values[0]
	else:
	# Spec'd length > 1 or undefined
	# Unspec'd, and length > 1
	dest[tag] = values

While I think this problem is worth fixing in it's own right, this does also resolve #2278. So if this is merged first, then #5425 becomes just about tidying up the tag value by turning it from a tuple into a bytestring.