Add validation for null members in structures, lists and maps

[maximk000]

Background

Previously, a NullNode could be provided as a required structure member without emitting a ValidationEvent. So an example test below would fail.

    public void nullRequiredStructureMember() {
        ObjectNode structure = ObjectNode.builder()
                .withMember("foo", Node.nullNode())
                .build();
        NodeValidationVisitor visitor = NodeValidationVisitor.builder()
                .value(structure)
                .model(MODEL)
                .allowOptionalNull(true)
                .build();
        List<ValidationEvent> events = MODEL
                .expectShape(ShapeId.from("ns.foo#Structure"))
                .accept(visitor);

        assertThat(events, hasSize(1));
        assertThat(events.get(0).getMessage(), containsString(
                "foo: Required member `foo` for structure ns.foo#Structure for cannot be null"));
    }

ns.foo#Structure schema:
"ns.foo#Structure": {
    "type": "structure",
    "members": {
        "foo": {
            "target": "ns.foo#String",
            "traits": {
                "smithy.api#required": {}
            }
        },
...

This happens because the memberShape function would check the member's target instead of itself, which always satisfies the !shape.isMemberShape() condition in the invalidShape function.

To fix this I added a new function to check null member nodes for structures, lists and maps. It relies on the isMemberNullable function, which checks that:

• If a structure member, member cannot be required
• If a list member, the list must have the sparse trait
• If a map value, the map must have the sparse trait (this case is not explicit but happens due to a fall through to the default case)

Testing

Added testcases:

• One that emits ValidationEvent when NullNode is provided as a required structure member
• One that emits ValidationEvent when NullNode is provided as member for a non-sparse list
• One that doesn't emit ValidationEvent when NullNode is provided as member for a sparse list
• One that emits ValidationEvent when NullNode is provided as a value for a non-sparse map
• One that doesn't emit ValidationEvent when NullNode is provided as a value for a sparse map

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[kstich]

Some comments on event message formatting (that will also require test updates) and a simplification of the conditional check.