Fix null/empty doc issue

Author: WolfgangSenff

addons/godot-firebase/firestore/firestore_document.gd

@@ -23,9 +23,10 @@ func _init(doc : Dictionary = {}):
 
 	if doc.has("fields"):
 		document = doc.fields
-	doc_name = doc.name
-	if doc_name.count("/") > 2:
-		doc_name = (doc_name.split("/") as Array).back()
+	if doc.has("name"):
+		doc_name = doc.name
+		if doc_name.count("/") > 2:
+			doc_name = (doc_name.split("/") as Array).back()
 
 	self.create_time = doc.createTime