When a bucket has default SSE-KMS encryption enabled and a file is uploaded
without explicit SSE headers, the encryption was applied correctly but the
SSE-KMS metadata (x-seaweedfs-sse-kms-key) was not serialized. This caused
downloads to fail with "empty SSE-KMS metadata" because the entry's Extended
map stored an empty byte slice.
The existing code already handled this for SSE-S3 bucket defaults
(SerializeSSES3Metadata) but was missing the equivalent call to
SerializeSSEKMSMetadata for the KMS path.
Fixesseaweedfs/seaweedfs#8776