Handle case where DAP line can be None

A comment in bugzilla pointed out a bug in my earlier patch to handle
the DAP "linesStartAt1" setting.  In particular, in the backtrace
code, "line" can be None, which would lead to an exception from
export_line.

This patch fixes the problem.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32468