Fix crash when GraphFrame sb_to_draw_panel is not a StyleBoxFlat

[Zylann]

Fixes #88014 (review)

This was flagged by GCC as a warning treated as error on my CI (not sure why Godot CI didnt catch it?).

scene/gui/graph_frame.cpp:121:84: error: 'this' pointer is null [-Werror=nonnull]
  121 |                                         sb_panel_texture = sb_panel_flat->duplicate();
      |                                                            ~~~~~~~~~~~~~~~~~~~~~~~~^~

[akien-mga]

Thanks!