Clarify Release serialization when optional fields are missing

[progval]

https://www.swhid.org/swhid-specification/v1.2/5.Core_identifiers/#55-releases says:

• author (arbitrary byte sequence, optional): generally contains the name and email address of the author of the release.
• author timestamp (decimal timestamp from the Unix epoch, optional): the date at which the release was authored.
• author timezone offset (arbitrary byte sequence, optional): UTC offset at which the release was authored, usually an ASCII-encoded [+/-]HHMM specification.

(emphasis mine)

and then says:

• if there is an author, the author line:
• the ASCII string "tagger" (6 bytes)
• an ASCII space
• the string of bytes provided for the author name and email, with each LF replaced by LF followed by an ASCII space
• an ASCII space
• the ASCII-encoded decimal representation of the author timestamp
• an ASCII space
• the string of bytes provided for the author timezone offset, with each LF replaced by LF followed by an ASCII space
• an LF

However:

• What should the timestamp and offset be replaced with if the author is present but not either of these fields?
• What to do if the timestamp and/or offset is present but not the author?