Fix CLI crash on a 3mf without project settings (#16004)

When a 3mf is tagged as written by OrcaSlicer or BambuStudio, the CLI
treats it as a project and reads the printer settings stored in the
file. #14580 guarded four of those reads, but printable_height still
called opt_float() without a check, so a 3mf whose
project_settings.config is empty or missing crashed with a null
dereference. The handy models OrcaBadge.3mf and OrcaSliced.3mf are both
like this.

Skip the read when the option is absent, like the reads around it. The
bed-size logic further down already treats an old printable height of 0
as unknown and uses the current printer's.

src/OrcaSlicer.cpp

@@ -1913,6 +1913,7 @@ int CLI::run(int argc, char **argv)
                         old_printable_width = static_cast<int>(old_printable_bbox.size().x());
                         old_printable_depth = static_cast<int>(old_printable_bbox.size().y());
                     }
+                    if (config.option<ConfigOptionFloat>("printable_height"))
                         old_printable_height = (int)(config.opt_float("printable_height"));
 
                     if (config.option<ConfigOptionFloat>("extruder_clearance_height_to_rod"))