Removing every layer from spec.layers now detaches them in AWS

[suprahh]

Issue #, if available: aws-controllers-k8s/community#3014

Description of changes

Removing every layer from spec.layers had no effect: the layers stayed
attached to the function in AWS while the resource was reported as synced.

Unlike #239, the delta is computed correctly here. The problem is in how the
update input is built: updateFunctionConfiguration only assigned
input.Layers inside if len(dspec.Layers) > 0, so once the desired spec had
no layer left the field stayed nil. UpdateFunctionConfiguration treats an
omitted Layers as "leave them as they are" and only detaches the existing
layers when it receives an empty list.

Layers is now always set, including when the list is empty.

Verification

Run against a real AWS account (us-west-2), with the controller running
locally against a kind cluster.

Before the fix, the new test fails: after emptying spec.layers the layer is
still attached.

tests/test_function.py::TestFunction::test_function_remove_layers FAILED
>       assert "Layers" not in function["Configuration"]
E       AssertionError: assert 'Layers' not in {'FunctionName': 'functionrmlayers-...', ...}

After the fix:

tests/test_function.py::TestFunction::test_function_remove_layers PASSED
1 passed, 19 deselected, 1 warning in 124.24s

test_function_layers still passes, so replacing layers with a different
non-empty list is unaffected.

This change is independent of #239 and can be merged in either order. I also
verified both branches merged together: test_function_layers,
test_function_add_layers_to_function_without_layers (from #239) and
test_function_remove_layers all pass on the combined tree.

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

[ack-prow]

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[ack-prow]

Hi @suprahh. Thanks for your PR.

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