SPWebSocketChannel: Setting lastChangeSignature on empty buckets

[jleandroperez]

Details:

In this PR we're setting an empty string, as the lastChangeSignature, whenever the indexed bucket is actually empty.

This will prevent constant reindex on empty buckets.

Testing:

1. Open the SimpletodoFinal app (samples folder)
2. Replace the SIMPERIUM_APP_ID and SIMPERIUM_API_KEY keys with a fresh app
3. Add, in this spot, the following snippet:

    self.simperium.verboseLoggingEnabled = true;

4. Launch the demo app and signup
5. Restart the app

Verify that after relaunching, the console won't show a reindex cycle anymore.

Needs review: @roundhill
Thanks in advance sir!

[roundhill]

LGTM

[jleandroperez]

Thanks for the review Dan!