Don't set erc-networks--id until network is known

Don't set erc-networks--id until network is known

* lisp/erc/erc-networks.el (erc-networks--id-given): Accept a null
argument.
(erc-networks--id-on-connect): Remove unused function.
(erc-networks--id-equal-p): Add method for comparing initialized and
unset IDs.
(erc-networks--update-server-identity): Ensure `erc-networks--id' is
set before continuing search.
(erc-networks--init-identity): Don't assume `erc-networks--id' is
non-nil.  Add branch condition to reload ID on non-nil case.
(erc-networks-on-MOTD-end): Let init-ID function handle renaming of
server buffer.

* lisp/erc/erc.el (erc-open): For continued sessions, try copying over
the last network ID.
(erc--auth-source-determine-params-default): Don't expect a network ID
to have been initialized.
(erc-set-current-nick): When connected, reload network ID.  Leave
comment warning that it may be unneeded.

* lisp/erc/erc-backend.el (erc-server-NICK, erc-server-433): Unless
already connected, schedule ID reload when server rejects or mandates
a nick change.

* test/lisp/erc/erc-scenarios-base-association-nick.el
(erc-scenarios-base-association-nick-bumped,
erc-scenarios-base-association-nick-bumped-mandated-renick): Update to
reflect more liberal association behavior when renamed by IRCd.