(PUP-2158) Default a missing manifest to no_manifest

The tests were often creating environments that didn't need to have
manifests. Since we didn't have a way of specifying that when they
were written, we used '' to fill in the blank. This actually caused a
large number of tests to try to find code to load in the PWD, which
caused tests to break if a developer had parse errors in manifests being
used for testing in the root directory of their puppet project. This
changes the manifest to be optional and removes '' from those tests. The
tests no longer fail if a bad manifest is in the PWD :D