Can't parse RFC822.TEXT NIL

[Davidde94]

RFC822.TEXT NIL is a valid message attribute, but for some reason we can't parse it.

[odrobnik]

This appears to have been fixed by #720 ("Allow NIL response for a particular body section", merged Oct 2022), which added:

• parseMessageAttribute_rfc822Text_nilBody and parseMessageAttribute_rfc822Header_nilBody handlers
• RFC822.TEXT / RFC822.HEADER entries in the parseMessageAttribute lookup table
• The nilBody(StreamingKind) case on MessageAttribute

I verified that both RFC822.TEXT NIL and RFC822.HEADER NIL parse correctly through the full ResponseParser pipeline on current main (added streaming test cases for both — they pass).

This issue can likely be closed.

[odrobnik]

@danieleggert I think you can close this.