[SPARK-48765][DEPLOY] Enhance default value evaluation for SPARK_IDENT_STRING

[pan3793]

What changes were proposed in this pull request?

This PR follows Hadoop[1] to enhance the SPARK_IDENT_STRING default value evaluation.

[1] https://github.com/apache/hadoop/blob/rel/release-3.4.0/hadoop-common-project/hadoop-common/src/main/bin/hadoop-functions.sh#L893-L896

Why are the changes needed?

I found in some cases, $USER is not available, thus the auto-generated log and pid file names are strange.

For example, there is no $USER when login to docker

$ docker run -t -i ubuntu:latest
root@1dbeaefd6cd4:/# echo $USER

root@1dbeaefd6cd4:/# id -nu
root
root@1dbeaefd6cd4:/# exit

Does this PR introduce any user-facing change?

Yes, affects log/pid file names.

How was this patch tested?

Manually tested.

Was this patch authored or co-authored using generative AI tooling?

No

[pan3793]

cc @HyukjinKwon @LuciferYang @yaooqinn

[LuciferYang]

I found in some cases, $USER is not available, thus the auto-generated log and pid file names are strange.

The code changes look okay, but can you give a stable bad case? and add it In pr description

[pan3793]

@LuciferYang one of the most common cases is docker env, I updated the PR description to show that.

[LuciferYang]

Merged into master. Thanks @pan3793