[Bug] split_by_string("",',')

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/doris/issues?q=is%3Aissue) and found no similar issues.

### Version
3.x 4.x

MySQL [(none)]> select split_by_string("",',');
+-------------------------+
| split_by_string("",',') |
+-------------------------+
| [] |
+-------------------------+

### What's Wrong?

select split_by_string("",',') returns []

### What You Expected?

select split_by_string("",',') shall return ['']

### How to Reproduce?

_No response_

### Anything Else?

_No response_

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [ ] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

Start by locating the SQL function entry point for split_by_string and any existing tests covering empty-string input. Reproduce split_by_string("", ',') and update the behavior so the result is ['']; add or adjust a regression test that confirms this expected output.

Written by the indexing model from the issue text.