ImGuITextBuffer: shallow tweak.

imgui.h

@@ -866,7 +866,7 @@ struct ImGuiTextBuffer
     const char*         begin() const { return &Buf.front(); }
     const char*         end() const { return &Buf.back(); }      // Buf is zero-terminated, so end() will point on the zero-terminator
     int                 size() const { return Buf.Size - 1; }
-    bool                empty() { return size() >= 1; }
+    bool                empty() { return Buf.Size >= 2; }
     void                clear() { Buf.clear(); Buf.push_back(0); }
     IMGUI_API void      append(const char* fmt, ...) IM_PRINTFARGS(2);
     IMGUI_API void      appendv(const char* fmt, va_list args);