Fix gc log variable judge in jdk8
### Motivation
The judge sentences of if is reversed, can not work in jdk8.
### Modifications
Fix the judge sentences.
@@ -49,7 +49,8 @@ PULSAR_GC=${PULSAR_GC:-"-XX:+UseG1GC -XX:MaxGCPauseMillis=10 -XX:+ParallelRefPro
# Garbage collection log.
IS_JAVA_8=`java -version 2>&1 |grep version|grep '"1\.8'`
-if [ ! $IS_JAVA_8 ]; then
+# java version has space, use [[ -n $PARAM ]] to judge if variable exists
+if [[ -n $IS_JAVA_8 ]]; then
PULSAR_GC_LOG=${PULSAR_GC_LOG:-"-Xloggc:logs/pulsar_gc_%p.log -XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+UseGCLogFileRotation -XX:NumberOfGCLogFiles=10 -XX:GCLogFileSize=20M"}
else
# After jdk 9, gc log param should config like this. Ignoring version less than jdk 8
