Dolt shell expects more input after empty line whereas MySQL shell returns nothing

MySQL
```
mysql>
mysql>
```

Dolt
```
tmp/main*>
->
```

Behavior is the same when the line contains only whitespaces (MySQL also expects more input)

No file or test is named. Start by locating the Dolt shell's input handling for blank lines and compare it with the shown MySQL behavior. Done means an empty line returns no continuation prompt, while whitespace-only input still requests more input; add or update a regression test for both cases.

Written by the indexing model from the issue text.