Bug fix: Inconsistent breakpoint setting if `tab_size` isn't been specified

For whatever reason, cloning the Package from GitHub left `tab_size` not specified until I opened and saved the settings file. It's happened before (perhaps due to installing while ST is running), but to get things working OK out of the box it required either:
  1. Adding ` .. or tab_size == None` to the default behaviour clause, **or**
  2. Opening and saving the settings

I don't know what I've done, but it was reproducible at work and home. :confused: