fix(format): guard against null members in FormatJvmModelInferrer.inferConstants

[joaodinissf]

The bug

FormatJvmModelInferrer.inferConstants adds createConstant(format, c) to the inferred JVM model unconditionally:

for (final Constant c : FormatGeneratorUtil.getAllConstants(format)) {
  it.getMembers().add(createConstant(format, c));   // createConstant can return null
}

createConstant returns null for a value-less constant (its if (stringValue) … else if (intValue) … return null fall-through — reachable for a constant in an incomplete/error state during editing). Adding that null to the JvmMember EList throws IllegalArgumentException: The 'no null' constraint is violated.

Root cause — a migration regression

The original Xtend was members += allConstants.map[createConstant], which compiles to JvmTypesBuilder.operator_add, and operator_add silently skips null elements. The Xtend→Java migration translated += to a bare loop-add, which does not skip nulls — so the null-skip semantics were lost. The sibling inferRules in the same file kept the null-skip, making this an inconsistent one-off.

The fix

Guard the add (equivalent to operator_add's null-skip / an IterableExtensions.filterNull):

final JvmMember member = createConstant(format, c);
if (member != null) {
  it.getMembers().add(member);
}

Verification

• New FormatJvmModelInferrerTest pins the behaviour: a value-less Constant must not yield a null member or throw.
• A/B proven through the full test harness: the test fails on the pre-fix code (IllegalArgumentException: The 'no null' constraint is violated) and passes with the guard. Full local gate: BUILD SUCCESS, 359 tests / 0 failures, all static analysis clean.

Found by an archaeology audit of the merged Xtend→Java migrations. The skill rule that prevents this class of defect is added in a separate PR (#1473, rules/10 §10.5 — operator_add skips nulls).

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