MM-13015 Remove metadata field from post when metadata is disabled (#9882)

model/post.go

@@ -81,7 +81,7 @@ type Post struct {
 	HasReactions  bool            `json:"has_reactions,omitempty"`
 
 	// Transient data populated before sending a post to the client
-	Metadata *PostMetadata `json:"metadata" db:"-"`
+	Metadata *PostMetadata `json:"metadata,omitempty" db:"-"`
 }
 
 type PostEphemeral struct {