### What changes were proposed in this pull request?
The PR checks for the emptiness of `--py-files` value and uses it only if it is not empty.
### Why are the changes needed?
There is a bug in Mesos cluster mode REST Submission API. It is using `--py-files` option without specifying any value for the conf `spark.submit.pyFiles` by the user.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
* Submitting an application to a Mesos cluster:
`curl -X POST http://localhost:7077/v1/submissions/create --header "Content-Type:application/json" --data '{
"action": "CreateSubmissionRequest",
"appResource": "file:///opt/spark-3.0.0-bin-3.2.0/examples/jars/spark-examples_2.12-3.0.0.jar",
"clientSparkVersion": "3.0.0",
"appArgs": ["30"],
"environmentVariables": {},
"mainClass": "org.apache.spark.examples.SparkPi",
"sparkProperties": {
"spark.jars": "file:///opt/spark-3.0.0-bin-3.2.0/examples/jars/spark-examples_2.12-3.0.0.jar",
"spark.driver.supervise": "false",
"spark.executor.memory": "512m",
"spark.driver.memory": "512m",
"spark.submit.deployMode": "cluster",
"spark.app.name": "SparkPi",
"spark.master": "mesos://localhost:5050"
}}'`
* It should be able to pick the correct class and run the job successfully.
Closes#29499 from farhan5900/SPARK-32675.
Authored-by: farhan5900 <farhan5900@gmail.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
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