fix(warn): avoid toRefs warning for Vue 2

Fix #648

If this PR vuejs/composition-api#821 gets fixed, this commit can be reverted

Author: stackmaster92

packages/pinia/src/store.ts

@@ -221,7 +221,9 @@ function createSetupStore<
   let debuggerEvents: DebuggerEvent[] | DebuggerEvent
   const initialState = pinia.state.value[$id] as UnwrapRef<S> | undefined
 
-  if (!initialState && (!__DEV__ || !hot)) {
+  // avoid setting the state for option stores are it is set
+  // by the setup
+  if (!buildState && !initialState && (!__DEV__ || !hot)) {
     if (isVue2) {
       set(pinia.state.value, $id, {})
     } else {