Warning on bulk read from MemcachedStorage

Version: 3.3.1

### Bug Description
When Memcached::getMulti() returns false intead of array of results, Warning is raised

### Steps To Reproduce
Use Memcached::getMulti() for example on server that doesnt respond, is down, etc.

### Expected Behavior
Same handling as in read() function. Return empty array like as nothing has been found

### Possible Solution
PR pending
