### What changes were proposed in this pull request?
A followup for 345d35ed1a, in this PR we support CURRENT_USER without tailing parentheses in default mode. And for ANSI mode, we can only use CURRENT_USER without tailing parentheses because it is a reserved keyword that cannot be used as a function name
### Why are the changes needed?
1. make it the same as current_date/current_timestamp
2. better ANSI compliance
### Does this PR introduce _any_ user-facing change?
no, just a followup
### How was this patch tested?
new tests
Closes#32770 from yaooqinn/SPARK-21957-F.
Authored-by: Kent Yao <yao@apache.org>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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