Increase flexibility when reading IPTC fields

[radarhere]

Resolves #7318

1. The issue has found an image where im.info["photoshop"][0x0404] appears to be zero padded. I've added a commit ignoring an IPTC field if it is only zero bytes, resolving the problem.
2. A later post in the issue has found an image where tag[0]
   

   Pillow/src/PIL/IptcImagePlugin.py

   Line 67 in 52206c7

   if s[0] != 0x1C or tag[0] < 1 or tag[0] > 9:

   
   is 240. This doesn't line up with page 12 (PDF page 12, not page 12 according to the footer) of http://www.iptc.org/std/IIM/4.1/specification/IIMV4.1.pdf, which lists 1 to 9. However, https://exiftool.org/TagNames/IPTC.html lists 240, making me think that this is a proprietary extension, rather than something mentioned in the format. I've adjusted the code to also accept 240 as a value.