fix(datasource/CloudSigma): Make sure public ssh key is not empty

Even when public ssh key is not set by the user, CloudSigma's server
context has a key `meta.ssh_public_key` which is just an empty string.
So instead of just relying on the "comma ok" idiom I make sure the value
is not an empty string.

datasource/metadata/cloudsigma/server_context.go

@@ -108,7 +108,7 @@ func (scs *serverContextService) FetchMetadata() (metadata datasource.Metadata,
 	}
 
 	metadata.SSHPublicKeys = map[string]string{}
-	if key, ok := inputMetadata.Meta["ssh_public_key"]; ok {
+	if key, _ := inputMetadata.Meta["ssh_public_key"]; len(key) > 0 {
 		splitted := strings.Split(key, " ")
 		metadata.SSHPublicKeys[splitted[len(splitted)-1]] = key
 	}