fix(Store): only default to initialValue when store value is undefined

[jbedard]

This assumes the store will only have an undefined state on initialization and sets the initialSate at that time. If the store state ever becomes undefined again in the future it will be reset to the initialState again. This basically means initialState is more of a "default state" then "initial", but this at least improves it to be an undefined-check instead of falsey-check. If we wanted to change this to be more strictly an "initial" state then this change would grow quite a bit.

For example if you add the following diff to the tests weird things occur:

@@ -74,8 +74,8 @@ describe('ngRx Store', () => {
       store = TestBed.get(Store);
       dispatcher = TestBed.get(ActionsSubject);
 
-      const actionSequence = '--a--b--c--d--e--f--g';
+      const actionSequence = '--a--b--c--d--e--f--g--h';
       const actionValues = {
         a: { type: INCREMENT },
         b: { type: 'OTHER' },
@@ -84,6 +84,7 @@ describe('ngRx Store', () => {
         e: { type: INCREMENT },
         f: { type: INCREMENT },
+        g: { type: CLEAR },   //set state to undefined
+        h: { type: 'OTHER' }, //reproduces https://github.com/ngrx/platform/issues/880
       };
       const counterSteps = hot(actionSequence, actionValues);
       counterSteps.subscribe(action => store.dispatch(action));

[coveralls]

Coverage remained the same at 92.87% when pulling 8deffc3 on jbedard:880 into cb173f6 on ngrx:master.

[MikeRyanDev]

Looks great! Thanks!

[jbedard]

Thanks for the quick merge.

Sorry I forgot to put "fixes #880" in the commit message though. Should #880 be closed? Or leave it open regarding the initial vs default issue?