[system tests] Validate transforms based on the mappings

internal/elasticsearch/client.go

@@ -361,6 +361,16 @@ func (c *Client) SimulateIndexTemplate(ctx context.Context, indexTemplateName st
 		return nil, nil, fmt.Errorf("error unmarshaling mappings: %w", err)
 	}
 
+	// In case there are no dynamic templates, set an empty array
+	if string(preview.Template.Mappings.DynamicTemplates) == "" {
+		preview.Template.Mappings.DynamicTemplates = []byte("[]")
+	}
+
+	// In case there are no mappings defined, set an empty map
+	if string(preview.Template.Mappings.Properties) == "" {
+		preview.Template.Mappings.Properties = []byte("{}")
+	}
+
 	return preview.Template.Mappings.DynamicTemplates, preview.Template.Mappings.Properties, nil
 }
 
@@ -403,5 +413,15 @@ func (c *Client) DataStreamMappings(ctx context.Context, dataStreamName string)
 		mappingsDefinition = v.Mappings
 	}
 
+	// In case there are no dynamic templates, set an empty array
+	if string(mappingsDefinition.DynamicTemplates) == "" {
+		mappingsDefinition.DynamicTemplates = []byte("[]")
+	}
+
+	// In case there are no mappings defined, set an empty map
+	if string(mappingsDefinition.Properties) == "" {
+		mappingsDefinition.Properties = []byte("{}")
+	}
+
 	return mappingsDefinition.DynamicTemplates, mappingsDefinition.Properties, nil
 }

internal/packages/packages.go

@@ -199,6 +199,9 @@ type TransformDefinition struct {
 	Source struct {
 		Index []string `config:"index" yaml:"index"`
 	} `config:"source" yaml:"source"`
+	Dest struct {
+		Index string `config:"index" yaml:"index"`
+	} `config:"dest" yaml:"dest"`
 	Meta struct {
 		FleetTransformVersion string `config:"fleet_transform_version" yaml:"fleet_transform_version"`
 	} `config:"_meta" yaml:"_meta"`