init: Allow -proxy="" setting values

This drops the `No proxy server specified. Use -proxy=<ip> or -proxy=<ip:port>`
error when a empty `-proxy=` command line argument, `bitcoin.conf` value, or
`settings.json` value is specified, and just makes bitcoin connect and listen
normally in these cases.

The error was originally added in https://github.com/bitcoin/bitcoin/pull/20003
to prevent a bare `-proxy` command line argument with no assignment from
clearing proxy settings. But it was implemented in an overbroad way breaking
empty `-proxy=` assignments as well.

The motivation for this change is to prevent a GUI bug that happens with
https://github.com/bitcoin/bitcoin/pull/15936, reported in
https://github.com/bitcoin/bitcoin/pull/15936#pullrequestreview-937685759 by
vasild, that happens after a proxy setting is enabled and disabled in the GUI.
But this change also makes sense on its own to remove a potentially confusing
error message.

src/util/system.h

@@ -175,6 +175,7 @@ public:
         // ALLOW_STRING = 0x08,   //!< unimplemented, draft implementation in #16545
         // ALLOW_LIST = 0x10,     //!< unimplemented, draft implementation in #16545
         DISALLOW_NEGATION = 0x20, //!< disallow -nofoo syntax
+        DISALLOW_ELISION = 0x40,  //!< disallow -foo syntax that doesn't assign any value
 
         DEBUG_ONLY = 0x100,
         /* Some options would cause cross-contamination if values for