* add test for importing empty .xsh file
* test: empty lines do not get appended to history
prompt-toolkit needs its own test outside of test_base_shell.py because it uses a custom _push() method
* fix: do not append empty/comment-only input to history
Adds a compile_empty_tree argument to Execer.compile()
By default, the argument is `True`, and `compile()` returns a compiled `pass` statement for comment-only input.
When the argument is `False`, `compile()` returns `None` for comment-only input.
The base shell and prompt-toolkit shell use `compile_empty_tree = False` so that they get `None` as the compiled code and don't append the command to the history.
* add news
* fix tests
