[SPARK-29574][K8S][FOLLOWUP] Fix bash comparison error in Docker entr…

…ypoint.sh

A small change to fix an error in Docker `entrypoint.sh`

When spark running on Kubernetes, I got the following logs:
```log
+ '[' -n ']'
+ '[' -z ']'
++ /bin/hadoop classpath
/opt/entrypoint.sh: line 62: /bin/hadoop: No such file or directory
+ export SPARK_DIST_CLASSPATH=
+ SPARK_DIST_CLASSPATH=
```
This is because you are missing some quotes on bash comparisons.

No

CI

Closes #28075 from dungdm93/patch-1.

Authored-by: Đặng Minh Dũng <dungdm93@live.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
(cherry picked from commit 1d0fc9a)
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

resource-managers/kubernetes/docker/src/main/dockerfiles/spark/entrypoint.sh

@@ -58,8 +58,8 @@ fi
 
 # If HADOOP_HOME is set and SPARK_DIST_CLASSPATH is not set, set it here so Hadoop jars are available to the executor.
 # It does not set SPARK_DIST_CLASSPATH if already set, to avoid overriding customizations of this value from elsewhere e.g. Docker/K8s.
-if [ -n ${HADOOP_HOME}  ] && [ -z ${SPARK_DIST_CLASSPATH}  ]; then
-  export SPARK_DIST_CLASSPATH=$($HADOOP_HOME/bin/hadoop classpath)  
+if [ -n "${HADOOP_HOME}"  ] && [ -z "${SPARK_DIST_CLASSPATH}"  ]; then
+  export SPARK_DIST_CLASSPATH="$($HADOOP_HOME/bin/hadoop classpath)"
 fi
 
 if ! [ -z ${HADOOP_CONF_DIR+x} ]; then