Do not try to base64 decode empty values

It is possible for consul kv items to be empty, e.g. you might get
something like

{"CreateIndex":915,"ModifyIndex":915,"LockIndex":0,"Key":"some/key/prefix","Flags":4,"Value":null}

In this case, node-consul will attempt to base64 decode this, which
results in an exception being thrown, e.g.:

TypeError: Cannot read property 'length' of null
    at new Buffer (buffer.js:184:31)
        at
        /proj/node_modules/consul-node/lib/kv.js:49:16
        ...

This prevents that from happening.