[lldb] Fix spurious assertion in PrintCommandOutput

When the string passed to PrintCommandOutput doesn't end with a newline,
`written` will exceed `size` and result in an lldbassert.

After 8e776bb6 we don't really need
written anymore and we can check whether `str` is empty instead. This
patch simplifies the code and removes the assert that's no longer
relevant.

Differential revision: https://reviews.llvm.org/D126081