Do not fail in case of empty image label values

[hisener]

If I understand correctly, image label value may be an empty string. However, in that case, I got "index out of range" errors in two places. This PR uses startswith to avoid them.

	File "/private/var/tmp/_bazel_halil.sener/f6fb7d40b6b3160ce57e2d52769a8d36/external/io_bazel_rules_docker/container/image.bzl", line 976, column 76, in container_image
		files = sorted({v[1:]: None for v in kwargs["labels"].values() if v[0] == "@"}.keys())
Error: index out of range (index is 0, but sequence has 0 elements)

	File "/private/var/tmp/_bazel_halil.sener/f6fb7d40b6b3160ce57e2d52769a8d36/external/io_bazel_rules_docker/container/image.bzl", line 199, column 17, in _image_config
		if fname[0] == "@":
Error: index out of range (index is 0, but sequence has 0 elements)

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

[github-actions]

This Pull Request has been automatically marked as stale because it has not had any activity for 180 days. It will be closed if no further activity occurs in 30 days.
Collaborators can add an assignee to keep this open indefinitely. Thanks for your contributions to rules_docker!