ytdl_hook: pass empty proxy value

an empty value to a commandline option usually means that parameter does
not expect anything, but in case of `--proxy`, yt-dlp use an empty value to
disable proxy setting, the empty value need to be passed to `--proxy`

player/lua/ytdl_hook.lua

@@ -954,7 +954,7 @@ local function run_ytdl_hook(url)
 
     for param, arg in pairs(raw_options) do
         table.insert(command, "--" .. param)
-        if arg ~= "" then
+        if arg ~= "" or param == "proxy" then
             table.insert(command, arg)
         end
         if (param == "sub-lang" or param == "sub-langs" or param == "srt-lang") and (arg ~= "") then