Respect a nil erc-session-password when reconnecting

Respect a nil erc-session-password when reconnecting

* lisp/erc/erc.el (erc-open): Simplify `old-vars' expression.
(erc--compute-server-password): Only compute a server password when
first connecting.  For compatibility, this respects third-party code
that expects session passwords in target buffers when initially
non-nil.
* test/lisp/erc/erc-scenarios-services-misc.el
(erc-scenarios-services-auth-source-reconnect): Add new test.
* test/lisp/erc/resources/services/auth-source/recon.eld: Add new test
data file.  (Bug#59858.)