[lldb/DWARF] Always construct a DWARFDebugInfo object

Change the return value of SymbolFileDWARF::DebugInfo from a pointer to
a reference, and remove all null checks.

Previously, we were not constructing the DebugInfo object when the
debug_info section was empty. Now we always construct the object but
it will return an empty list of dwarf units (a thing which it already
supported).