[LLDB] Fix completion of space-only lines in the REPL on Linux (#83203)

https://github.com/modularml/mojo/issues/1796 discovered that if you try
to complete a space-only line in the REPL on Linux, LLDB crashes. I
suspect that editline doesn't behave the same way on linux and on
darwin, because I can't replicate this on darwin.

Adding a boundary check in the completion code prevents the crash from
happening.