Fixed the empty string issue when decoding MBULK_REPLY

#### Motivations:
A `$0\r\n\r\n` being one element of a `MBULK_REPLY` from a Redis server should be decoded to a `BulkReply` of `""`, not a `EmptyBulkReply `. 

#### Modifications:

1. Fixed the `if` statement to catch `$0\r\n\r\n`

2. And the tests in both NaggatiSpec.scala and ClientSpec.scala

#### Results:
Now we can have `$0\r\n\r\n` being properly decoded.