[TypeScript] Fix some strict null checks errors in core

[fzaninotto]

Fixes the following TS compilation errors when enabling strickNullChecks:

     3  src/auth/useGetIdentity.ts:51
     2  src/auth/useHandleAuthCallback.ts:15
     1  src/core/useResourceDefinitions.ts:24
     1  src/dataProvider/fetch.ts:103
     1  src/form/useUnique.ts:109

In master, this reduces the number of strict null check errors from 246 to 238.

Refs #9622