Handle cache miss response in MultiStore

What does this MR do and why?

This MR addresses the inconsistent behaviour or MultiStore when dealing with cache misses (empty response). redis-rb client returns several forms of "empty" responses

• nil for operations like get. This is a falsey
• true/false for operations like sismember and exists. The value directly translates to truthy/falsey.
• 0 for count-based lookups like smembers, hlen or -2 for ttl. This is a truthy.
• [nil, nil...] for operations like mget. This is a truthy.
• [] for operations like smembers. This is a truthy.
• {} for operations like hgetall. This is a truthy.
• [0, [] for operations like hscan, sscan. This is a truthy.

However, MultiStore will perform a fallback_read only for nil and false responses since the other forms of responses are truthy.

See #386978 (closed) for more background.

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