Fix Firestore document caching so it works (#461)

Author: WolfgangSenff

addons/godot-firebase/firestore/firestore_collection.gd

@@ -22,8 +22,6 @@ var _base_url : String
 var _extended_url : String
 var _config : Dictionary
 
-var _documents := {}
-
 # ----------------------- Requests
 
 ## @args document_id
@@ -44,11 +42,16 @@ func get_doc(document_id : String, from_cache : bool = false, is_listener : bool
 	_process_request(task, document_id, url)
 	var result = await Firebase.Firestore._handle_task_finished(task)
 	if result != null:
+		var found_document = false
 		for child in get_children():
 			if child.doc_name == document_id:
 				child.replace(result, true)
 				result = child
+				found_document = true
 				break
+		
+		if not found_document:
+			add_child(result, true)
 	else:
 		print("get_document returned null for %s %s" % [collection_name, document_id])
 
@@ -96,7 +99,7 @@ func update(document : FirestoreDocument) -> FirestoreDocument:
 	var temp_transforms
 	if document._transforms != null:
 		temp_transforms = document._transforms
-		document._transforms = null
+		document._transforms = FieldTransformArray.new()
 
 	var body = JSON.stringify({"fields": document.document})