Error using Gazelle extension with --incompatible_disallow_empty_glob enabled

[cgrindel]

Error Message

ERROR: Traceback (most recent call last):
	File "/private/var/tmp/_bazel_chuck/91330c3c30ebaed046a35357439e8c08/external/bazel_skylib_gazelle_plugin/bzl/BUILD", line 28, column 13, in <toplevel>
		] + glob([
Error in glob: glob pattern 'testdata/**' didn't match anything, but allow_empty is set to False (the default value of allow_empty can be set with --incompatible_disallow_empty_glob).
ERROR: /Users/chuck/code/cgrindel/bazel-starlib/renovate/bazel_skylib-1.x/BUILD.bazel:15:15: Target '@bazel_skylib_gazelle_plugin//bzl:bzl' contains an error and its package is in error and referenced by '//:gazelle_bin'

It looks like the Gazelle extension is packaged without its test data. This glob causes the error when --incompatible_disallow_empty_glob is enabled. I suspect that we just need to explicitly set allow_empty = True in that glob.

[cgrindel]

FYI. I am going to put up a PR in a moment.