For some reason strings are sometimes passed to `findUnmet()`, which
is obviously unexpected given the way the code is written. Rather than
figure out why strings are passed and how to safely avoid passing
strings, just return early. The net effect is the same, but returning
early avoids setting a property on a string, which is prohibited in
strict mode.
The debug statement mostly printed the following useless message over
and over, causing Travis CI logs to become truncated:
[DEBUG] pluginfw - [ undefined ] returning
