persist: On error deserializing, don't store null

We don't use these nulls in any later handling; they go into the
REHYDRATE payload, but then in redux-persist-migrate we cast them
away to pretend they can't be there.  It's not clear how we would
usefully do anything with them, either.

Instead, on an error here just omit that key, the same as we would
if it weren't there in the underlying storage at all.

This lets us cut out a fixme step in redux-persist-migrate.  The type
of the REHYDRATE payload is still a lie, but it's now the same lie on
its way into this code as out of it.

Author: gnprice

src/actionTypes.js

@@ -106,13 +106,12 @@ import type {
  *
  * @prop payload A version of the global Redux state, as persisted by the
  *     app's previous runs.  This will be empty on first startup or if the
- *     persisted state is just missing keys, and will have `null` at each
- *     key where an error was encountered in reading the persisted state.
- *     In any case it will only contain the keys we configure to be persisted.
+ *     persisted state is just missing keys.  In any case it will only
+ *     contain the keys we configure to be persisted.
  */
 type RehydrateAction = $ReadOnly<{|
   type: typeof REHYDRATE,
-  +payload: $ReadOnly<$ObjMap<$Rest<GlobalState, { ... }>, <V>(V) => V | null>>,
+  +payload: $ReadOnly<$Rest<GlobalState, { ... }>>,
 |}>;
 
 type AppOnlineAction = $ReadOnly<{|

src/redux-persist-migrate/index.js

@@ -25,10 +25,7 @@ export default function createMigration(
 
   const migrationDispatch = next => (action: Action) => {
     if (action.type === REHYDRATE) {
-      /* $FlowIgnore[incompatible-type]
-         this really is a lie -- and kind of central to migration */
-      const incomingState: State = action.payload;
-      return next({ ...action, payload: migratePayload(incomingState) });
+      return next({ ...action, payload: migratePayload(action.payload) });
     }
     return next(action);
   };

src/third/redux-persist/getStoredState.js

@@ -48,7 +48,6 @@ export default async function getStoredState(config: Config): Promise<{ ... }> {
             message: 'redux-persist/getStoredState: Error restoring data for a key.',
             key,
           });
-          restoredState[key] = null;
         }
       })(),
     ),