DWARFContext: Return empty data extractors instead of null pointers

Summary:
There are several reasons for doing this:
- generally, there's no reason to differentiate between a section being
  absent and it being present, but empty
- it matches more closely what llvm DWARF parser is doing (which also
  doesn't differentiate the two cases)
- SymbolFileDWARF also doesn't differentiate the two cases, which makes
  porting the rest of sections easier
- it fixes a bug in how the return-null-if-empty logic was implemented
  (it returned nullptr only the second time we tried to get the
  debug_aranges section), which meant that we hit an assert when trying
  to parse an empty-but-present section

Reviewers: JDevlieghere, clayborg, aprantl

Subscribers: zturner, lldb-commits

Differential Revision: https://reviews.llvm.org/D61942

llvm-svn: 360874