[yaml2obj] Fixing opening empty yaml files.

Essentially echo "" | yaml2obj crashes. This patch attempts to trim whitespace
and determine if the yaml string in the file is empty or not. If the input is
empty then it will not properly print out an error message and return an error
code.

Differential Revision: https://reviews.llvm.org/D59964

A    test/tools/yaml2obj/empty.yaml
M    tools/yaml2obj/yaml2obj.cpp

llvm-svn: 357219