Fix parsing of empty matchobject fields by the redis PB marshaling/unmarshaling code about open-match HOT 1 CLOSED

These errors are coming from two places:

• https://github.com/GoogleCloudPlatform/open-match/blob/master/internal/statestorage/redis/redispb/redispb.go#L159
• https://github.com/GoogleCloudPlatform/open-match/blob/master/internal/statestorage/redis/redispb/redispb.go#L167

One case when errors happen there is when pbMap doesn't have a key: Sprintf'ed string will not be valid JSON then.

While working on this I noticed weird issue at first sight: Wrapping these places with error checks makes Backend API to start sending an empty MatchObject in a response to profile without "rosters" field set. After some debugging I think there's a bug in this same function that breaks Watcher:

• When Evaluator approves proposal it renames the key in Redis.
• To detect a fact of proposal approval the Backend's Watcher periodically calls UnmarshalFromRedis() which sends HGETALL under hood.
• According to Redis docs HGETALL returns empty list when key does not exist, and Redigo seem to not treat empty list as error. And this breaks the logic of Watcher's work since UnmarshalFromRedis() doesn't check for results' length. (It was working previously because of this "invalid character '}'" JSON bug which caused UnmarshalFromRedis() to return error making Watcher think that there's not results yet)

from open-match.