Default to showing the latest block if no height provided

#### What's New
If no value is entered in the "Block Height" input field, default to grabbing data for the latest block.

#### Issues
* It may not be the most efficient idea to call ```getinfo();``` but it still shouldn't be called more than once on page-load, so this isn't really an issue anyway. Just raising this so we can tidy/move this code later if we need to.