fix(store): StoreOptions.defaults should not be required (#3499)

.changes/store-optional-defaults.md

@@ -0,0 +1,6 @@
+---
+store: patch
+store-js: patch
+---
+
+Fix `StoreOptions` requires `defaults` field

plugins/store/guest-js/index.ts

@@ -21,7 +21,7 @@ export type StoreOptions = {
   /**
    * Default value of the store
    */
-  defaults: { [key: string]: unknown }
+  defaults?: { [key: string]: unknown }
   /**
    * Auto save on modification with debounce duration in milliseconds, it's 100ms by default, pass in `false` to disable it
    */