Don't send empty lines for implicit targets in ERC

Don't send empty lines for implicit targets in ERC

* erc.el (erc-send-input-line): Previously, any line typed into a
query or channel buffer without an explicit user-command
handler (meaning most lines), would be sent twice because a trailing
newline (linefeed) would be appended. This has been verified by
checking IRCd server logs. IRCds won't return an error upon receiving
an empty message, but they also won't forward them to channel
subscribers and DM pals.

* erc-tests.el: Add test for erc-process-input-line, which also
indirectly tests erc-send-input-line. It also tests the command
lookup and dispatch facility (bug#50008).