Fix misc bugs in debugging features 

Bugs fixed:
- Zero-length function names result in errors logged to server (reported by JJ)
- Breakpoints can't be set in functions that have raw NA values in their parse trees (reported by William Hughes)
- The wrong function gets set as current in some Shiny debug sessions (reported by Winston)
- Refreshing the browser while calls are active results in debug mode turning on even when there's no browser() (reported by JJ)
- Errors in the logs when the code model is missing `findNextSignificantToken` (reported by JJ)