Allow printing even if nozzle info isn't reported by firmware. (#12814)

* Don't assume 0.4mm nozzle; format as "unknown" if not defined.

* Skip nozzle diameter and hardness checks if nozzle info unknown.

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Co-authored-by: SoftFever <softfeverever@gmail.com>

src/slic3r/GUI/GUI_Utils.hpp

@@ -66,6 +66,7 @@ wxDECLARE_EVENT(EVT_VOLUME_DETACHED, VolumeDetachedEvent);
 #endif /* _WIN32 */
 
 wxTopLevelWindow* find_toplevel_parent(wxWindow *window);
+wxString format_nozzle_diameter(float diameter);
 
 void on_window_geometry(wxTopLevelWindow *tlw, std::function<void()> callback);