Normalize ISUPPORT params with empty values in ERC

Normalize ISUPPORT params with empty values in ERC

* lisp/erc/erc-backend.el (erc-server-parameters)
(erc--isupport-params): Mention parsing and storage behavior regarding
nonstandard "FOO=" tokens.
(erc--parse-isupport-value): Move comment closer to code.
(erc--get-isupport-entry): Treat the empty string as truly null, as
prescribed by the Brocklesby draft cited in the top-level comment.
* test/lisp/erc/erc-tests.el (erc--get-isupport-entry): Add case for
the empty string appearing as a value for an `erc-server-parameters'
item.
(erc-server-005): Assert compat-related behavior of retaining the
empty string as a valid value from a raw "FOO=" token.
(Bug#67220)

(cherry picked from commit 25d15391f2683ea95c4d7ee291fb82e0c9858d73)