Properly handles possible nil values

[farazdagi]

What type of PR is this?

Other/Cleanup

What does this PR do? Why is it needed?

• Properly handles issues with improper handling of nil values

Other notes for review

• Issue was detected when reviewing the following code:
  

  prysm/validator/db/db.go

  Lines 69 to 71 in ac862d1

  	kv, err := NewKVStore(dirPath)
  	// Initialize the required public keys into the DB to ensure they're not empty.
  	if err := kv.initializeSubBuckets(pubKeys); err != nil {

  
  Here we do not properly handle error and possible nil value returned for the kv object.
• Couple of issues of the same origin have been identified in other places and fixed as well.

[nisdas]

nice catches