escape/alt-enter no longer runs multi-line query in safe mode

[owst]

Description

On master (d5cdd2ad4ea5c70dae766f2e00a71510e9c76b58) escape/alt-enter no longer run a multi-line query when in safe mode, but instead inserts a new line. This seems to have originated in https://github.com/dbcli/pgcli/pull/1065 - as @jonathanslenders suggests there, the change overrides the prompt_toolkit behaviour of accepting input when in multi-line mode, which breaks the safe mode of multi_line_mode introduced in https://github.com/dbcli/pgcli/pull/577 (as there is no way to accept the input and run the query)