feat: add generate_empty_layers attribute to js_image_layer

[gregmagolan]

Useful for when using js_image_layer with rules_docker in consistent manner for all images regardless of which layers are empty:

RULES_JS_LAYERS = [
    "node",
    "package_store_3p",
    "package_store_1p",
    "node_modules",
    "app",
]

def js_container_layers(name, binary, root, tags = []):
    js_image_layer(
        name = "{}/layers".format(name),
        binary = binary,
        generate_empty_layers = True,
        root = root,
        visibility = ["//visibility:__pkg__"],
    )

    [
        [
            native.filegroup(
                name = "{}/layers/{}.tar".format(name, layer),
                srcs = ["{}/layers".format(name)],
                output_group = layer,
            ),
            container_layer(
                name = "{}/{}_layer".format(name, layer),
                tars = [":{}/layers/{}.tar".format(name, layer)],
                tags = tags + ["manual"],
            ),
        ]
        for layer in RULES_JS_LAYERS
    ]

with generate_empty_layers = False the above example may fail with a message such as:

(18:01:23) ERROR: /home/runner/work/rules_js/rules_js/e2e/js_image_docker/BUILD.bazel:78:16: in tars attribute of container_layer_ rule //:package_store_1p_layer: '//:package_store_1p_tar' does not produce any container_layer_ tars files (expected .tar, .tar.xz, .tar.gz or .tgz). Since this rule was created by the macro 'container_layer', the error might have been caused by the macro implementation

if there are empty layers

[aspect-workflows]

Test

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//docs:update_5_test [k8-fastbuild]                 134ms

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