fix: tarball unbound variable error if zero tags provided #1003

Job	Run time
matrix-prep-os	1s
matrix-prep-bazelversion	3s
test-auth	1m 57s
test (ubuntu-latest, 6.2.0, ., true)	4m 32s
test (ubuntu-latest, 6.2.0, ., false)	1m 23s
test (ubuntu-latest, 6.2.0, e2e/custom_registry, false)	36s
test (ubuntu-latest, 6.2.0, e2e/wasm, false)	3m 51s
test (ubuntu-latest, 6.2.0, e2e/smoke, true)	3m 36s
test (ubuntu-latest, 6.2.0, e2e/smoke, false)	1m 36s
test (ubuntu-latest, 6.2.0, e2e/crane_as_registry, true)	3m 23s
test (ubuntu-latest, 6.2.0, e2e/crane_as_registry, false)	2m 11s
test (ubuntu-latest, 5.3.2, ., false)	2m 6s
test (ubuntu-latest, 5.3.2, e2e/custom_registry, false)	41s
test (ubuntu-latest, 5.3.2, e2e/wasm, false)	1m 30s
test (ubuntu-latest, 5.3.2, e2e/smoke, false)	4m 2s
test (ubuntu-latest, 5.3.2, e2e/crane_as_registry, false)	3m 12s
test (macos-latest, 6.2.0, ., true)	4m 17s
test (macos-latest, 6.2.0, ., false)	8m 16s
test (macos-latest, 6.2.0, e2e/smoke, true)	47s
test (macos-latest, 6.2.0, e2e/smoke, false)	41s
	48m 41s

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