Send shutdown and exit messages without arguments

Send shutdown and exit messages without arguments

Fix regression introduced in 70e6157b (https://github.com/joaotavora/eglot/issues/315).  According to the
LSP specification the exit notification and the shutdown request
shouldn't have arguments ("params: void").  Note that jsonrpc.el
send nil as null on the wire.

* eglot.el (eglot-shutdown): Change back the arguments of
:shutdown and :exit to nil.

GitHub-reference: fix https://github.com/joaotavora/eglot/issues/430