Fix Drawing.undo() crash on empty or single-element drawings

[leofernandezg]

Summary

• Fixes crashes in Drawing.undo() when the drawing has zero or one element

Problem

undo() had three failure modes:

1. Empty drawing: self.elements.pop(-1) raises IndexError
2. Single element: after pop, self.elements[-1].absdrop raises IndexError on empty list
3. Non-interactive mode: self.fig.clear() raises AttributeError because fig is None

Changes

• schemdraw/schemdraw.py: Guard against empty element list (early return), reset position to origin when all elements are removed, skip figure operations when fig is None
• test/test_undo.py: New test file covering empty drawing, single element, and undo-then-add scenarios

Test results

Without fix: 3 of 3 tests fail
With fix: 3 of 3 tests pass

Fixes #94

[leofernandezg]

Test suite results

Full test suite (py.test --nbval-lax test/) run on this branch produces the same results as master:

Branch	Failed	Passed
master (baseline)	38	432
this branch	38	435

The 38 pre-existing failures are in test_parselogic.ipynb and test_timing.ipynb (unrelated to this change).

[leofernandezg]

Update: Re-ran the full test suite with all optional dependencies installed (matplotlib, ziamath, pyparsing).

Branch	Passed	Failed
master	470	0
fix/undo-empty-drawing	473	0

No regressions introduced.