Fall back to default SSL provider for unmapped SNI

The RSocket WebSocket transport's HttpServerSslCustomizer kept its own
copy of the SNI provider lookup that returned null for server names
without a mapped SSL bundle. This commit adapts the copy to fall back
to the default provider, as it is done in gh-50301.

See gh-50640

Signed-off-by: Lee JiWon <dlwldnjs1009@gmail.com>