fix: allow copy_to_directory to have an empty srcs list

[gregmagolan]

Requested by a Bazel support customer where a copy_to_directory target in their repo had no srcs.

Remove an explicit fail check on empty srcs since outputting an empty directory is valid in Bazel and a valid use case for copy_to_directory rule.

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Test plan

• New test cases added

[aspect-workflows]

Test

1 test target passed

Targets

//lib/tests/copy_to_directory:case_22_test [k8-fastbuild] 126ms

Total test execution time was 126ms. 217 tests (99.5%) were fully cached saving 44s.

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Test

e2e/copy_action

All tests were cache hits

1 test (100.0%) was fully cached saving 73ms.

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Test

e2e/copy_to_directory

All tests were cache hits

6 tests (100.0%) were fully cached saving 583ms.

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Test

e2e/coreutils

All tests were cache hits

4 tests (100.0%) were fully cached saving 423ms.

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Test

e2e/external_copy_to_directory

All tests were cache hits

1 test (100.0%) was fully cached saving 73ms.

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Test

e2e/smoke

All tests were cache hits

6 tests (100.0%) were fully cached saving 2s.

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Buildifier      Format      Gazelle