Make hints @nonnull

The reason we couldn't do this previously is that we were setting `hints` to
`null` in the `PublicKeyCredentialRequestOptions` generator. We no longer do
since commit 89e78ff, so we can now require
this to be non-null.

Author: emlun

webauthn-server-core/src/main/java/com/yubico/webauthn/data/PublicKeyCredentialCreationOptions.java

@@ -333,7 +333,7 @@ public PublicKeyCredentialCreationOptionsBuilder hints(
           Arrays.stream(hints).map(PublicKeyCredentialHint::getValue).toArray(String[]::new));
     }
 
-    public PublicKeyCredentialCreationOptionsBuilder hints(List<String> hints) {
+    public PublicKeyCredentialCreationOptionsBuilder hints(@NonNull List<String> hints) {
       this.hints = hints;
       return this;
     }

webauthn-server-core/src/main/java/com/yubico/webauthn/data/PublicKeyCredentialRequestOptions.java

@@ -230,7 +230,7 @@ public PublicKeyCredentialRequestOptionsBuilder hints(
           Arrays.stream(hints).map(PublicKeyCredentialHint::getValue).toArray(String[]::new));
     }
 
-    public PublicKeyCredentialRequestOptionsBuilder hints(List<String> hints) {
+    public PublicKeyCredentialRequestOptionsBuilder hints(@NonNull List<String> hints) {
       this.hints = hints;
       return this;
     }