tpm2-util: do not serialize tpm2 bank if none is specified

tpm2-util: do not serialize tpm2 bank if none is specified

If both literal and signed PCR bindings are not used then we won't
determine a PCR bank to use, and hence we shouldnt attempt to serialize
it either.

Hence, if the bank is zero, skip serialization.

(And while we are at it, also skip serialization of the primary
algorithm if not set, purely to make things systematic).

[This effectively results in little change, as previously we'd then
seralize a json "null", while now we simply won't genreate the field]