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.

索引モデルが issue の本文から書いたものです。