Let send_nsca(8) ignore empty input lines

A command such as the following triggered an "Input format incorrect"
message:

	printf 'jupiter\tdisk\t0\okay\n\27\n' | send_nsca

This happened because the trailing newline character was interpreted as
another check result.  In practice, such a trailing newline character
might be generated unintentionally when using echo(1) to pipe check
results into the client.  Therefore, send_nsca(8) now ignores empty
input lines.