Allow parsing when there are no functional groups

[dspadea]

Currently receiving error on a response I'm getting from a vendor. The message contains only an ISA, TA1, and IEA segments.

"unable to enqueue generic segment when no functional groups have been added"

Even though the message may not be "complete" (they are a response to a known-invalid test case), it would still be nice to be able to parse these segments so we can determine what happened.

[sezna]

That use case makes sense, I'd rather this functionality be parameterized as it would technically be an invalid document if there is no functional group enclosing it.