Fix Span#pretty_print for empty duration

[marcotc]

This PR ensures Span#pretty_print is able to handle all valid values it prints.
The required changes were around the span's timing values.

This PR also adds a smoke test to ensure Span#pretty_print at least outputs a string without any errors.

The main error we stated seeing

     NoMethodError:
       undefined method `-' for nil:NilClass
     # ./lib/ddtrace/span.rb:392:in `duration`

was actually addressed in a previous PR by @ericmustin, but I remove a few of the safe-guard rescue statements from def duration when making changes to improve performance. I seek forgiveness with this PR 🙇.

[marcotc]

These were always wrong, as start_time in start_time = start_time.to_f references the newly declared variable, which it was always nil. It now references self.start_time, the instance value.