riemann: Skip empty fields

When adding properties to an event, skip empty fields before trying to
do any type casting. This allows metric(int("${foo:-}")
state("${state}")) to work correctly, and skip the metric field if it is
empty, and therefore fixes #258.

Reported-by: Fabien Wernli
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>