net: Print error message if -proxy is specified without arguments (instead of continuing without proxy server)

src/init.cpp

@@ -1176,6 +1176,10 @@ bool AppInitParameterInteraction(const ArgsManager& args)
 
     nMaxTipAge = args.GetArg("-maxtipage", DEFAULT_MAX_TIP_AGE);
 
+    if (args.IsArgSet("-proxy") && args.GetArg("-proxy", "").empty()) {
+        return InitError(_("No proxy server specified. Use -proxy=<ip> or -proxy=<ip:port>."));
+    }
+
     return true;
 }