Detect blank variable in trueorfalse

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
diff --git a/tests/test_truefalse.sh b/tests/test_truefalse.sh
index 2689589..03996ce 100755
--- a/tests/test_truefalse.sh
+++ b/tests/test_truefalse.sh
@@ -8,6 +8,14 @@
 source $TOP/functions
 source $TOP/tests/unittest.sh
 
+# common mistake is to use $FOO instead of "FOO"; in that case we
+# should die
+bash -c "source $TOP/functions-common; VAR=\$(trueorfalse False \$FOO)" &> /dev/null
+assert_equal 1 $? "missing test-value"
+
+VAL=$(trueorfalse False MISSING_VARIABLE)
+assert_equal "False" $VAL "blank test-value"
+
 function test_trueorfalse {
     local one=1
     local captrue=True