Allow empty mappings for optional YAML input
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