gdb/dap: check for breakpoint source before unpacking

Not all breakpoints have a source location. For example, a breakpoint
set on a raw address will have only the "address" field populated, but
"source" will be None, which leads to a RuntimeError when attempting to
unpack the filename and line number.

Before attempting to unpack the filename and line number from the
breakpoint, ensure that the source information is not None. Also
populate the source and line information separately from the
"instructionReference" field, so that breakpoints that include only an
address are still included.

Approved-By: Tom Tromey <tom@tromey.com>