Handle empty lambda expressions in UnitsCompiler (fix #248)

[dyrpsf]

Fixes #248.

Deriving units for a lambda expression while its child list is still empty
(e.g., immediately after the lambda node is constructed, before addChild()
has been called) could cause an IndexOutOfBoundsException in
UnitsCompiler.lambda(List<ASTNode2>).

Previously, lambda(...) always did:

values.get(values.size() - 1)

without checking whether values was empty, so an empty child list led to a
call to values.get(-1).

Changes

1. Updated UnitsCompiler.lambda(List values) to guard against an empty list:

   • If values is null or empty, the method now returns invalid(), i.e., an ASTNode2Value whose UnitDefinition is marked as INVALID. This avoids out-of-bounds access while signalling that units for a bare lambda expression (without arguments/body yet) are unknown at that time.

2. For non-empty values, behaviour is unchanged:

   • All but the last child are treated as arguments and stored in namesToUnits.

   • The units of the last child (the body) are used as the lambda’s result units.

Tests

1. Added UnitsCompilerLambdaTest under core/test/org/sbml/jsbml/math/compiler:

   • testLambdaWithNoChildrenDoesNotThrow():

     • Calls new UnitsCompiler(3, 1).lambda(Collections.<ASTNode2>emptyList()).

     • Asserts that no IndexOutOfBoundsException is thrown.

     • Asserts that the returned units are marked as invalid.

2. Ran:

mvn -pl core -Dtest=UnitsCompilerLambdaTest test
mvn -pl core test

Both complete with BUILD SUCCESS on Java 11.