[10.x] Handle empty arrays for DynamoDbStore multi-key operations

[nie7321]

When using the DynamoDB cache driver, if someone passes an empty list of keys to Cache::many([]) or an empty key/value array to Cache::putMany([]), DynamoDB will give an error response and the call will throw an exception. For example:

InvalidArgumentException: Found 1 error while validating the input provided for the BatchGetItem operation:
[RequestItems][vapor_cache][Keys] expected list element count to be >= 1, but found list element count of 0

This behaviour is unlike other cache drivers, which do not throw exceptions. For example:

>>> Cache::store('database')->many([])
=> []

>>> Cache::store('database')->putMany([])
=> true

That resulted in a papercut since I'm using the db driver for development and dynamo when deploying to vapor.

This PR adds a check for the array size and returns early if there's no Dynamo call to make.