0.23.0 container_bundle crashes when called with empty images list

[hanikesn]

🐞 bug report

Affected Rule

The issue is caused by the rule: `incremental_load`

Is this a regression?

Yes, the previous version in which this bug was not present was: ....

Introduced via #1957, confirmed working 0.22

Description

Using `container_bundle` with an empty `images = {}` causes an error

🔬 Minimal Reproduction

load("@io_bazel_rules_docker//container:container.bzl", "container_bundle")

container_bundle(
    name = "bundle_to_push",
    images = {}
    tags = ["no-remote-cache"],
)

🔥 Exception or Error

Traceback (most recent call last):
        File "/home/hanikel/.cache/bazel/186c259db594aa2669550cf1f9e672c6/external/io_bazel_rules_docker/container/bundle.bzl", line 67, column 15, in _container_bundle_impl
                _incr_load(
        File "/home/hanikel/.cache/bazel/186c259db594aa2669550cf1f9e672c6/external/io_bazel_rules_docker/container/layer_tools.bzl", line 232, column 28, in incremental_load
                run_tag = images.keys()[0]
Error: index out of range (index is 0, but sequence has 0 elements)