Return nil from more ERC response handlers

Return nil from more ERC response handlers

* etc/ERC-NEWS: Mention that certain aberrant response handlers now
return nil.
* lisp/erc/erc-backend.el (define-erc-response-handler): Mention that
body should explicitly return nil.
(erc-server-PART)
(erc-server-PING): Return nil.
* lisp/erc/erc-sasl.el (erc-sasl--destroy): Return nil.
* lisp/erc/erc.el (erc-display-message): Mention in doc string that
the return value is undefined.
(erc-kill-channel-hook): Fix package-version.
* test/lisp/erc/erc-networks-tests.el (erc-networks--set-name): Ensure
`erc--route-insertion' returns nil because this influences whether
response-handler hooks continue running.
* test/lisp/erc/erc-sasl-tests.el
(erc-sasl-create-client-ecdsa): Fix regression that made test
unusable, although it's still relatively useless and therefore skipped
by default.
* test/lisp/erc/erc-services-tests.el
(erc-services-tests--auth-source-standard)
(erc-services-tests--auth-source-announced): Clarify annotations.
* test/lisp/erc/erc-tests.el (erc-message): Don't return non-nil in
mocked `erc-display-message'.
(erc-send-modify-hook): Shadow `erc-send-modify-hook' because
`erc-stamp--date-mode' modifies it locally.