There is no guarantee that this environment variable is set. Eg, when
running a test outside of the build system, such as under a debugger.
And passing a nullptr to the string constructor is undefined.
Use an empty string, which seems like it is close to the original
intent.
This patch adds targets for printf and fprintf to the bazel build.
Additionally, it adds support for the build system to specify where
files should be written for testing purposes. This was necessary to
enable the fprintf test under bazel.
Reviewed By: sivachandra
Differential Revision: https://reviews.llvm.org/D147008
