Fix empty JSON encrypted content parsing

[hej090224]

Fixes #1035

Summary

This updates JWT parsing so encrypted content with an empty JSON object byte payload ({}) is not automatically treated as Claims unless Claims parsing was explicitly requested.

The previous behavior caused parseEncryptedContent to fail with Unexpected Claims JWE. when arbitrary encrypted content happened to deserialize as an empty JSON object.

Changes

• Preserve empty JSON object payloads as content for content parsing paths.
• Keep Claims parsing behavior when Claims are explicitly requested.
• Add a regression test for encrypted content containing {} bytes.

Verification

• .\mvnw -pl impl -am -Dtest=JwtsTest '-Dsurefire.failIfNoSpecifiedTests=false' test
• .\mvnw -pl impl -am -Dtest=JwtParserTest '-Dsurefire.failIfNoSpecifiedTests=false' test
• .\mvnw verify