Fix introspection for abstract classes (#10889)

[rorueda]

Ignore constructors for abstract classes, so the introspection is written as not buildable and the instantiation methods in the introspection are not overridden (trying to call a constructor for an abstract class).

[dstepanov]

Doesn't look like a correct fix. This should be fixed in IntrospectedTypeElementVisitor

[graemerocher]

yes this fix is not in the correct place, though the tests add value

[rorueda]

Thanks for the pointers, @dstepanov. I'll look into it.

Should the abstract verification below be removed in favor of the fix on IntrospectedTypeElementVisitor? I was unsure of the semantic of getDefaultConstructor, and because of this, I ended up thinking it was to consider only constructors that can be directly called.

micronaut-core/inject-groovy/src/main/groovy/io/micronaut/ast/groovy/visitor/GroovyClassElement.java

Lines 342 to 349 in dfe9b0f

	private Optional<MethodElement> possibleDefaultEmptyConstructor() {
	List<ConstructorNode> constructors = classNode.getDeclaredConstructors();
	if (CollectionUtils.isEmpty(constructors) && !classNode.isAbstract() && !classNode.isEnum()) {
	// empty default constructor
	return createMethodElement(new ConstructorNode(Modifier.PUBLIC, new BlockStatement()));
	}
	return Optional.empty();
	}