Fix segfault in --merge CLI command (#14548)

[joshherkness]

Summary

Running PrusaSlicer --merge model.stl results in a segmentation fault. The root cause is an unsafe access to the dont_arrange option - calling transform.opt_bool("dont_arrange") when the option hasn't been set causes the crash.

Changes

• Added safety check before accessing the dont_arrange option
  • If the transform does not have dont_arrange, we'll skip checking the dont_arrange option since this causes crash

Testing

• ✅ ~/Projects/prusa-slicer/build/src/PrusaSlicer --merge /tmp/3DBenchy.stl /tmp/3DBenchy.stl --export-stl --output /tmp/merged.stl, verify no crash occurs and output contains merged models which have been arranged

# - ✅ `~/Projects/prusa-slicer/build/src/PrusaSlicer --merge /tmp/3DBenchy.stl /tmp/3DBenchy.stl --export-stl --output /tmp/merged.stl --dont-arrange`, verify no crash occurs and output contains merged models which have **not** been arranged

Issue

• PrusaSlicer CLI on MacOS (ARM) segmentation faults on --merge flag #14548