Hanndle empty short output more sensibly.

When the first line of output contained perfdata but no short output,
the perfdata portion of the line was used as the short output. The
short output is now taken to be an empty string in these cases.

Cases where the first output line is entirely empty (no whitespace)
are still handled as "(No output returned from plugin)", whether or
not additional lines follow.

These situations are unusual, and likely indicate a misbehaving check,
but are not disallowed by our documented output format.

Additionally: avoid no-op iterations of the line processing loop on
empty input or end of input; and clarify/cleanup some comments.