Skip trying to create empty manifest tar

If there are no manifests we generate the following errors:

tar: Cowardly refusing to create an empty archive
Try 'tar --help' or 'tar --usage' for more information.
command terminated with exit code 123
tar: This does not look like a tar archive
tar: Exiting with failure status due to previous errors
I, [2022-03-21T20:10:18.349339 compumike#1]  INFO -- : No JSONs to retrieve. Skipping

Skip calling tar if no manifests are found.

src/main.rb

@@ -116,7 +116,7 @@ def fetch_manifests
     @_manifest_tmpdir = File.join("/dev/shm", SecureRandom.hex)
     FileUtils.mkdir(@_manifest_tmpdir)
 
-    Open3.pipeline(["kubectl", "exec", "-n", TROW_NAMESPACE, TROW_POD, "--", "/bin/bash", "-c", "find /data/manifests/ -type f -print0 | xargs -0 tar -cf -"],
+    Open3.pipeline(["kubectl", "exec", "-n", TROW_NAMESPACE, TROW_POD, "--", "/bin/bash", "-c", "find /data/manifests/ -type f -print0 | xargs --no-run-if-empty -0 tar -cf -"],
       ["tar", "-C", @_manifest_tmpdir, "-xf", "-"])
   end