Previously, in the `OcdbtCoordinatorResource::Spec`, the
`RpcSecurityMethod` pointer was inconsistently either `nullptr` or a
pointer to the insecure security singleton object.  Attempting to
convert a spec to JSON with a null `RpcSecurityMethod` pointer led to
a crash.

PiperOrigin-RevId: 534655231
Change-Id: Ibe45a0b44b8a268b708f1ab96d19705f96777d1f