[SPARK-3319] [SPARK-3338] Resolve Spark submit config paths

The bulk of this PR is comprised of tests. All changes in functionality are made in `SparkSubmit.scala` (~20 lines).

**SPARK-3319.** There is currently a divergence in behavior when the user passes in additional jars through `--jars` and through setting `spark.jars` in the default properties file. The former will happily resolve the paths (e.g. convert `my.jar` to `file:/absolute/path/to/my.jar`), while the latter does not. We should resolve paths consistently in both cases. This also applies to the following pairs of command line arguments and Spark configs:

- `--jars` ~ `spark.jars`
- `--files` ~ `spark.files` / `spark.yarn.dist.files`
- `--archives` ~ `spark.yarn.dist.archives`
- `--py-files` ~ `spark.submit.pyFiles`

**SPARK-3338.** This PR also fixes the following bug: if the user sets `spark.submit.pyFiles` in his/her properties file, it does not actually get picked up even if `--py-files` is not set. This is simply because the config is overridden by an empty string.