bug: Empty (or short?) table headers causes crash

[tingham]

When specifying a table with:

{
 ...
 "headers": [],
  "data": []
}

We get crashing issues when calculating the frame dimensions in the draw method.

Expected behavior What you expected to happen instead.

Probably good if it didn't / doesn't crash.

Reproduction Code to reproduce the behavior (optional).

I'll provide a PR when I have something that seems right that resolves the issue.