Need to cross check `isFullyEncrypted` impl

[tomholub]

Need to cross check this isFullyEncrypted - for one thing there is a dummy gnu S2K requiring special treatment, second there could be some null-like encryption method like plain. (sometimes the spec is messy). That will be for another PR.

Originally posted by @tomholub in #1085 (comment)

[DenBond7]

pgpainless/pgpainless#99

[IvanPizhenko]

PGPainless has made a change
pgpainless/pgpainless@d082f12

[IvanPizhenko]

Should be already covered in the #1216