fix(common/providers/database): DataSnapshot.val() throwing error when source is null (#1682)

Author: CorieW

src/common/providers/database.ts

@@ -126,6 +126,9 @@ export class DataSnapshot implements database.DataSnapshot {
   val(): any {
     const parts = pathParts(this._childPath);
     let source = this._data;
+    if (source === null) {
+      return null;
+    }
     if (parts.length) {
       for (const part of parts) {
         if (source[part] === undefined) {