repair optional parameter

tasks/mergefq.wdl

@@ -9,10 +9,11 @@ task mergefq {
 
 	command <<<
 		set -euo pipefail
-		if [  ${fq3} != "" ]; then
-		cat ${fq1} ${fq2} > ${fq_m}
+		if [  ${fq3} = "" ]
+		then
+			cat ${fq1} ${fq2} > ${fq_m}
 		else
-		cat ${fq1} ${fq2} ${fq3} > ${fq_m}
+			cat ${fq1} ${fq2} ${fq3} > ${fq_m}
 		fi
 	>>>