Describe the bug

The repeat scalar function currently panics causing a capacity overflow error for negative numbers.

To Reproduce

SELECT repeat('hi', -1)
thread 'main' panicked at library/alloc/src/slice.rs:503:50:
capacity overflow
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Expected behavior

Postgres and Spark both appear to return empty strings when presented with a negative number.

postgres=# SELECT LENGTH(repeat('hi', -1));
 length 
--------
      0
(1 row)

Additional context

No response