Fix nil check for RNDSupporter

[fhofherr]

The initial implementation suffered from Go's slightly peculiar handling
of nil interfaces.

Basically if a variable of an interface type gets assigned a pointer to
a nil value, the variable is not nil:

var rdns hcloud.RDNSSupporter

// returns nil as first return value
rdns, _, err = client.Server.GetByID(id)
if rnds == nil {
    // Never reached as rdns is not nil.
}

See also https://go.dev/doc/faq#nil_error

Fixes #482