tests: check how command works with invalid input

Lack of this check resulted to bug that was shipped to users, which was
fixed in commit 805d353.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>

tests/Makefile.am

@@ -1,6 +1,7 @@
 TESTS = \
 	bootp \
 	complete \
+	empty \
 	full-json \
 	leading0 \
 	same-twice \

tests/empty

@@ -0,0 +1 @@
+test.sh

tests/expected/empty

@@ -0,0 +1,9 @@
+Ranges:
+shared net name     first ip           last ip            max   cur    percent  touch   t+c  t+c perc
+
+Shared networks:
+name                   max   cur     percent  touch    t+c  t+c perc
+
+Sum of all ranges:
+name                   max   cur     percent  touch    t+c  t+c perc
+All networks             0     0       -nan       0      0      -nan