Tighten up the yamilizer so it stops eliding empty sequences if the embedded empty sequence is the first key/value in a map which is itself in a sequence.

Patch with help from Nick Kledzik.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188508 91177308-0d34-0410-b5e6-96231b3b80d8