[SPARK-19707][CORE] Improve the invalid path check for sc.addJar

## What changes were proposed in this pull request?

Currently in Spark there're two issues when we add jars with invalid path:

* If the jar path is a empty string {--jar ",dummy.jar"}, then Spark will resolve it to the current directory path and add to classpath / file server, which is unwanted. This is happened in our programatic way to submit Spark application. From my understanding Spark should defensively filter out such empty path.
* If the jar path is a invalid path (file doesn't exist), `addJar` doesn't check it and will still add to file server, the exception will be delayed until job running. Actually this local path could be checked beforehand, no need to wait until task running. We have similar check in `addFile`, but lacks similar similar mechanism in `addJar`.

## How was this patch tested?

Add unit test and local manual verification.

Author: jerryshao <sshao@hortonworks.com>

Closes #17038 from jerryshao/SPARK-19707.