[test] Fix append_path in the empty case

Summary:
normpath() was being called on an empty string and appended to
the environment variable in the case where the environment variable
was unset. This led to ":." being appended to the path, since
normpath() of an empty string is '.', presumably to represent cwd.

Reviewers: zturner, sqlbyme, modocache

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38542

llvm-svn: 314915