Merge bitcoin/bitcoin#24830: init: Allow -proxy="" setting values

1d4122dfef init: Allow -proxy="" setting values (Ryan Ofsky)

Pull request description:

  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.

ACKs for top commit:
  hebasto:
    re-ACK 1d4122dfef, only rebased since my recent [review](https://github.com/bitcoin/bitcoin/pull/24830#pullrequestreview-941255672).

Tree-SHA512: 753adfce199ed078a6cd9e0ea78e76c0b14070f8fcfe2a4632cd0c6dfe6b4e135ddffbe11a97e5e30520ea9e5bda00bad1493cbaef74cf425aa8613249167f53

test/functional/feature_config_args.py

@@ -85,7 +85,7 @@ class ConfArgsTest(BitcoinTestFramework):
 
     def test_invalid_command_line_options(self):
         self.nodes[0].assert_start_raises_init_error(
-            expected_msg='Error: No proxy server specified. Use -proxy=<ip> or -proxy=<ip:port>.',
+            expected_msg='Error: Error parsing command line arguments: Can not set -proxy with no value. Please specify value with -proxy=value.',
             extra_args=['-proxy'],
         )