check for empty cached poll data to prevent warning on load of editor

[jgcaruso]

This fixes a warning in the editor that was found while testing the v1.7.1 plugin release

Testing

1. apply pr
2. run make client (maybe not necessary?)
3. add a /poll to a post or page
4. Save then reload the page
5. You should NOT see a warning appear "behind" the editor as it is loading that an array key does not exist