nvmf_tcp: Don't require a data digest for PDUs without data

If a PDU (such as a Command Capsule PDU) on a connection that has
enabled data digests does not have a data section, it will not have
the the PDU data digest flag set. The previous check was requiring
this flag to be present on all PDU types that support data sections
even if no data was included in the PDU.

Sponsored by: Chelsio Communications