Merge pull request #1020 from sigstore/rekor-empty-set

Fix check for empty SET in `RekorVerifier`

Author: aaronlew02

sigstore-java/src/main/java/dev/sigstore/rekor/client/RekorVerifier.java

@@ -71,7 +71,8 @@ public void verifyEntry(RekorEntry entry) throws RekorVerificationException {
                     new RekorVerificationException(
                         "Log entry (logid) does not match any provided transparency logs."));
 
-    if (entry.getVerification().getSignedEntryTimestamp() != null) {
+    var set = entry.getVerification().getSignedEntryTimestamp();
+    if (set != null && !set.isEmpty()) {
       try {
         var verifier = Verifiers.newVerifier(tlog.getPublicKey().toJavaPublicKey());
         if (!verifier.verify(