fix(scripts/cassandra-harry): Fixing bash if condition

The correct way to check if variable is set is:
```bash
if [ -z ${var+x} ]; then
   echo "var is unset"
else
   echo "var is set to '$var'"
fi
```

For more information please check the following link:
https://stackoverflow.com/questions/3601515/how-to-check-if-a-variable-is-set-in-bash
diff --git a/scripts/cassandra-harry b/scripts/cassandra-harry
index 94ee991..7cd4592 100755
--- a/scripts/cassandra-harry
+++ b/scripts/cassandra-harry
@@ -1,6 +1,6 @@
 #!/bin/sh
 
-if [ "x$HARRY_HOME" == 'x' ]; then
+if [ -z ${HARRY_HOME1+x} ]; then
     HARRY_HOME=~/cassandra-harry
 fi
 
@@ -41,14 +41,13 @@
     esac
 done
 
-
-if [[ ! -z $node ]]; then
+if [ ! -z ${node+x} ]; then
     sed -i -e "s/contact_points:.*/contact_points: $node/g" $HARRY_HOME/conf/external.yaml
 fi
-if [[ ! -z $run_time ]]; then
+if [ ! -z ${run_time+x} ]; then
     sed -i -e "s/run_time:.*/run_time: $run_time/g" $HARRY_HOME/conf/external.yaml
 fi
-if [[ ! -z $run_time_unit ]]; then
+if [ ! -z ${run_time_unit+} ]; then
     sed -i -e "s/run_time_unit:.*/run_time_unit: \"$run_time_unit\"/g" $HARRY_HOME/conf/external.yaml
 fi