Don't let #{application,framework,full}_trace be nil

Those three can be nil when exception backtrace is nil. This happens and
that forced a couple of nil guards in the code. I'm proposing to make
those always return an array, even on nil backtrace.
[CI SKIP] Missing space.
Merge pull request #17636 from gsamokovarov/exception-wrapper-less-nils

Don't let #{application,framework,full}_trace be nil
Merge pull request #17635 from tgxworld/missing_space

[ci skip] Missing space in Active Job guide for referring to Action Mailer
Rename #source_extract to #source_extracts in ExceptionWrapper

It returns multiple source extracts since 1ed264bc. Also cleaned its
result structure, as we no longer need the file in a code extract.