Fix lines in scope widget go under a wrong group

[ktakiman]

Fix #888. When dap.ui.Layer.render is called with start == nil and end_ == nil, assign start = 0 only when the buffer contains a single, empty line. For all other cases, assign start == line-count instead of start == line-count - 1, which appears to be causing the line swapping issue.
Add a simple test case where the buffer already contains non-empty lines, and a new line is added with start == nil and end_ == nil, which should now be appended to the end.

[mfussenegger]

Thanks

[ktakiman]

Thank YOU for the great plugin 👍, and accepting my PR 😃