LibWeb: Implement PBKDF2 getKeyLength for SubtleCrypto

Author: https://github.com/stelar7
Commit: abf55fe33d
Pull-request: https://github.com/SerenityOS/serenity/pull/23834
Reviewed-by: https://github.com/ADKaster ✅

Userland/Libraries/LibWeb/Crypto/CryptoAlgorithms.cpp

@@ -1397,4 +1397,10 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<JS::ArrayBuffer>> PBKDF2::derive_bits(Algor
     return JS::ArrayBuffer::create(realm, result.release_value());
 }
 
+WebIDL::ExceptionOr<JS::Value> PBKDF2::get_key_length(AlgorithmParams const&)
+{
+    // 1. Return null.
+    return JS::js_null();
+}
+
 }