### What changes were proposed in this pull request?
Spark's CSV source can optionally ignore lines starting with a comment char. Some code paths check to see if it's set before applying comment logic (i.e. not set to default of `\0`), but many do not, including the one that passes the option to Univocity. This means that rows beginning with a null char were being treated as comments even when 'disabled'.
### Why are the changes needed?
To avoid dropping rows that start with a null char when this is not requested or intended. See JIRA for an example.
### Does this PR introduce _any_ user-facing change?
Nothing beyond the effect of the bug fix.
### How was this patch tested?
Existing tests plus new test case.
Closes#29516 from srowen/SPARK-32614.
Authored-by: Sean Owen <srowen@gmail.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
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