BUGFIX: Fix TypeError in RedisBackendTest
BUGFIX: Fix TypeError in RedisBackendTest

Some tests failed with

    Return value of Neos\Cache\Backend\RedisBackend::isFrozen() must
    be of the type boolean, null returned

after the introduction of return type declarations.

fcool/flow-development-collection,kdambekalns/flow-development-collection,dlubitz/flow-development-collection,gerhard-boden/flow-development-collection,dlubitz/flow-development-collection,aertmann/flow-development-collection,fcool/flow-development-collection,kdambekalns/flow-development-collection,bwaidelich/flow-development-collection,albe/flow-development-collection,robertlemke/flow-development-collection,bwaidelich/flow-development-collection,neos/flow-development-collection,robertlemke/flow-development-collection,gerhard-boden/flow-development-collection,neos/flow-development-collection,aertmann/flow-development-collection,neos/flow-development-collection,robertlemke/flow-development-collection,kdambekalns/flow-development-collection,daniellienert/flow-development-collection,dlubitz/flow-development-collection,kdambekalns/flow-development-collection,daniellienert/flow-development-collection,albe/flow-development-collection,gerhard-boden/flow-development-collection,fcool/flow-development-collection,daniellienert/flow-development-collection,robertlemke/flow-development-collection,fcool/flow-development-collection,neos/flow-development-collection,aertmann/flow-development-collection,bwaidelich/flow-development-collection,albe/flow-development-collection