[BUG] raw kv scan cannot start with empty start key

[birdstorm]

Describe the bug

In ScanIterator.java, the start key cannot be empty.

if (startKey.isEmpty()) {
  throw new IllegalArgumentException("start key cannot be empty");
} 

thus if RawKV wants to scan from the very beginning of all data, the ScanIterator will throw an exception.

Java Client and TiDB/TiKV version info

master