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)

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What is the current behavior?

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Does this PR introduce a breaking change?

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