As a follow-on to I8cefb58f49dcd2cb2def8a5071d0892af520e7f7, put in
some detection around missing variable-to-test arguments in
trueorfalse.
Correct a couple of places where we were passing in blank strings,
resulting in the default always being applied.
Add test-cases and enhance the documentation a little.
Depends-On: I8cefb58f49dcd2cb2def8a5071d0892af520e7f7
Change-Id: Icc0eb3808a2b6583828d8c47f0af4181e7e2c75a
e82bac0498