Handle simple strings given as seriesinfo

[cabo]

https://github.com/ietf-tools/xml2rfc/issues/1301#issuecomment-3457780592

The example given:

   seriesinfo:
     ITU-T Recommendation X.690

should probably simply be translated to

   refcontent:
     ITU-T Recommendation X.690

(There also needs to be some handling for the explicitly empty value= case, which may be the same.)