Bench: Empty files fix

benchs/Makefile

@@ -28,7 +28,7 @@ all: ${OUTPUT_LTL} ${OUTPUT_REACH} ${OUTPUT}
 
 ${OUTPUT}: ${LTLGLOBAL} ${REACHGLOBAL}
 	@echo "file,transpile_time, transpile_memory, states, transitions, time, memory" > $@
-	@if [ -f $^ ]; then\
+	@if [ ! -z $^ ]; then\
 		tail -q -n +2 $^ >> $@;\
 	fi
 
@@ -39,7 +39,7 @@ blackbox: ${OUTPUT_BB}
 
 ${OUTPUT_BB}: ${LTLBB}
 	@echo "model,transpile_time,transpile_memory,states,transitions,blackbox_spot_time,blackbox_ltsmin_time" > $@
-	@ if [ -f $< ]; then\
+	@if [ ! -z $< ]; then\
 		tail -q -n +2 $< >> $@;\
 	fi
 
@@ -47,7 +47,8 @@ ${LTLBB}: ${LTLOUTPUT}
 
 ${OUTPUT_LTL}: ${LTLOUTPUT}
 	@echo "model,isempty,spottime,ltsmintime,formulae" > $@
-	@if [ -f $< ]; then\
+	@if [ ! -z $< ]; then\
+		echo here;\
 		tail -q -n +2 $< >> $@;\
 	fi
 
@@ -68,7 +69,7 @@ ${LTLOUTPUT}: %-ltl/${OUTPUT_LTL}: RERS/%-ltl.go.formulae.txt RERS/%-ltl.go %-lt
 
 ${OUTPUT_REACH}: ${REACHOUTPUT}
 	@echo "model,isempty,spottime,ltsmintime,formulae" > $@
-	@if [ ! -f $< ]; then\
+	@if [ ! -z $< ]; then\
 		tail -q -n +2 $< >> $@;\
 	fi