[spire-server] x509pop .TrustDomain is missing

[kfox1111]

https://github.com/spiffe/spire/blob/main/doc/plugin_server_nodeattestor_x509pop.md#agent-path-template
Says that .TrustDomain should be valid.

But, it is missing from:

spire/pkg/common/plugin/x509pop/x509pop.go

Lines 270 to 275 in 77e2c2a

	agentPath, err := agentPathTemplate.Execute(agentPathTemplateData{
	Certificate: cert,
	PluginName: PluginName,
	SerialNumberHex: SerialNumberHex(cert.SerialNumber),
	Fingerprint: Fingerprint(cert),
	})