fix: handle nil backtrace

[3v0k4]

• Story: https://trello.com/c/fRBnccqI/486-bugknapsackpro-gem-fix-an-error-when-a-backtrace-is-nil-in-log-threads
• Issue: log threads unable to give the backtrace #270

Description

#backtrace could be nil in:

• Exception
• Thread

Better to look at the c source as the docs may not be always clear on what the return values are (as in this case).

I don't think we need additional test coverage because we are not dealing with a bug that could cause a regression.

Checklist reminder

• You added the changes to the UNRELEASED section of the CHANGELOG.md, including the needed bump (ie, patch, minor, major)
• You follow the architecture outlined below for RSpec in Queue Mode, which is a work in progress (feel free to propose changes):
  • Pure: lib/knapsack_pro/pure/queue/rspec_pure.rb contains pure functions that are unit tested.
  • Extension: lib/knapsack_pro/extensions/rspec_extension.rb encapsulates calls to RSpec internals and is integration and e2e tested.
  • Runner: lib/knapsack_pro/runners/queue/rspec_runner.rb invokes the pure code and the extension to produce side effects, which are integration and e2e tested.