wayland: pass actual empty strings to zenity

The double quotes were passed literally to the zenity arguments which
resulted in the message box displaying literal `""` when no text was
given. The empty string is more logical in this case, e.g., the empty
title results in the message box having lesser height.

src/video/wayland/SDL_waylandmessagebox.c

@@ -123,14 +123,14 @@ bool Wayland_ShowMessageBox(const SDL_MessageBoxData *messageboxdata, int *butto
         argv[argc++] = "--title";
         argv[argc++] = messageboxdata->title;
     } else {
-        argv[argc++] = "--title=\"\"";
+        argv[argc++] = "--title=";
     }
 
     if (messageboxdata->message && messageboxdata->message[0]) {
         argv[argc++] = "--text";
         argv[argc++] = messageboxdata->message;
     } else {
-        argv[argc++] = "--text=\"\"";
+        argv[argc++] = "--text=";
     }
 
     for (i = 0; i < messageboxdata->numbuttons; ++i) {
@@ -143,7 +143,7 @@ bool Wayland_ShowMessageBox(const SDL_MessageBoxData *messageboxdata, int *butto
             argv[argc++] = "--extra-button";
             argv[argc++] = messageboxdata->buttons[i].text;
         } else {
-            argv[argc++] = "--extra-button=\"\"";
+            argv[argc++] = "--extra-button=";
         }
     }
     if (messageboxdata->numbuttons == 0) {