[firestore] add support for `DocumentSnapshot.get('dot.notated.field.path')`

lib/modules/firestore/DocumentSnapshot.js

@@ -63,6 +63,7 @@ export default class DocumentSnapshot {
     if (fieldPath instanceof FieldPath) {
       return extractFieldPathData(this._data, fieldPath._segments);
     }
-    return this._data ? deepGet(this._data, fieldPath, '.') : undefined;
+
+    return deepGet(this._data, fieldPath, '.');
   }
 }

lib/utils/index.js

@@ -25,6 +25,7 @@ export function deepGet(
   path: string,
   joiner?: string = '/'
 ): any {
+  if (!isObject(object)) return undefined;
   const keys = path.split(joiner);
 
   let i = 0;