Fix margins on icon-only messages

Messages with an icon and no text (such as in the game start sequence) had an oversized margin due to ImGui adding padding for an empty string.

Source/Core/VideoCommon/OnScreenDisplay.cpp

@@ -116,6 +116,7 @@ static float DrawMessage(int index, Message& msg, const ImVec2& position, int ti
     }
 
     // Use %s in case message contains %.
+    if (msg.text.size() > 0)
       ImGui::TextColored(ARGBToImVec4(msg.color), "%s", msg.text.c_str());
     window_height =
         ImGui::GetWindowSize().y + (WINDOW_PADDING * ImGui::GetIO().DisplayFramebufferScale.y);