udf test for `unix_timestamp("")` might fail

```log
/home/runner/work/OpenMLDB/OpenMLDB/hybridse/src/udf/udf_test.h:172: Failure
Expected equality of these values:
expect
Which is: Nullable{value=1674015278, type=int64}
output
Which is: Nullable{value=1674015279, type=int64}
```

refer https://github.com/4paradigm/OpenMLDB/actions/runs/3945482097/jobs/6752411020#step:7:21561

Fail might happen due to slow execution.

The failing test is in hybridse/src/udf/udf_test.h at line 172, testing the unix_timestamp function with an empty string. The test expects a specific timestamp but gets a slightly different one, likely due to timing. Start by examining the test logic and the UDF implementation for unix_timestamp. Run the test locally to reproduce the failure, then consider mocking time or adjusting the test tolerance. 'Done' means the test passes consistently.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.