fix(shaclgen): emit sh:maxCount 0 for zero maximum_cardinality

[jdsika]

Summary

Fix a Python truthiness bug in the SHACL generator that prevents sh:maxCount 0 and sh:minCount 0 from being emitted when maximum_cardinality: 0 or minimum_cardinality: 0 is set in a LinkML schema.

Problem

In shaclgen.py, the cardinality checks use Python truthiness:

if s.minimum_cardinality:    # 0 is falsy in Python!
    prop_pv_literal(SH.minCount, s.minimum_cardinality)
...
if s.maximum_cardinality:    # 0 is falsy in Python!
    prop_pv_literal(SH.maxCount, s.maximum_cardinality)

Since 0 evaluates as False in Python, setting maximum_cardinality: 0 (which should emit sh:maxCount 0 meaning "this property MUST NOT appear") produces no output at all.

Root Cause

The condition if s.maximum_cardinality: fails when the value is 0 because Python treats 0 as falsy. The correct check is if s.maximum_cardinality is not None: which distinguishes "not set" from "explicitly set to zero".

Fix

Changed both checks to use explicit is not None comparisons:

if s.minimum_cardinality is not None:
    prop_pv_literal(SH.minCount, s.minimum_cardinality)
...
if s.maximum_cardinality is not None:
    prop_pv_literal(SH.maxCount, s.maximum_cardinality)

This matches the pattern already used in the OWL generator (owlgen.py lines 627-640) for the same attributes.

Verification

• W3C SHACL spec explicitly allows sh:maxCount 0 (means "property must not exist on any conforming node")
• OWL generator already correctly uses is not None and emits owl:maxCardinality 0
• docgen.py also uses is not None for the same field (line 693)
• Added regression test that verifies sh:maxCount 0 appears in generated output

Use Case

This is needed for modeling class hierarchies where subclasses restrict inherited properties. For example, slot_usage with maximum_cardinality: 0 is the idiomatic way in LinkML to express "this inherited slot is not applicable on this subclass" --- but without this fix, the SHACL output silently omits the constraint.

Testing

• Added ChildWithZeroMaxCard class to tests/linkml/test_generators/input/shaclgen/cardinality.yaml
• Added test_zero_maximum_cardinality_emits_maxcount regression test to test_shaclgen.py
• Existing tests continue to pass (non-zero cardinalities unaffected by is not None check)

Note on exact_cardinality

The elif s.exact_cardinality: branches (lines 174, 184) have the same truthiness issue for the value 0. However, exact_cardinality: 0 is semantically degenerate (a list with exactly zero items is the same as a forbidden property) and extremely unlikely in practice. This fix focuses on the common and semantically meaningful case. A follow-up can address exact_cardinality if needed.