Allow empty mappings for optional YAML input

Summary:
This change fixes a bug where `obj2yaml` can in some cases produce YAML that
causes `yaml2obj` to error.

The ELF YAML document structure has a `Sections` mapping, which contains three
mappings, all of which are optional: `Local`, `Global`, and `Weak.` Any one of
these can be missing, but if all three are missing, then `yaml2obj` errors. This
change allows YAML input for cases like this one.

I have tested this with check-llvm and check-lld, and all tests passed.

This change is the result of test failures while working on D39582, which
introduces a `DynamicSymbols` mapping, which will be empty at times.

Reviewers: compnerd, jakehehrlich, silvas, kledzik, mehdi_amini, pcc

Reviewed By: compnerd

Subscribers: silvas, llvm-commits

Differential Revision: https://reviews.llvm.org/D39908

llvm-svn: 318428