minimal/do: 'redo-ifchange' with no parameters shouldn't try to build all.

It should just build nothing.  Because sometimes you want to do something
like:
	redo-ifchange $(find -name '*.c')

And the find doesn't return any results.  This is consistent with what real
redo does.

Added a test to confirm that it works.

t/blank/blank.do

@@ -0,0 +1,2 @@
+redo-ifchange
+

t/test.do

@@ -4,4 +4,4 @@ redo nonshelltest shelltest \
 	deltest deltest2 test.args test2.args passfailtest chdirtest \
 	curse/test deps/test "space dir/test" modetest makedir2 \
 	silencetest touchtest stamp/test alwaystest ifcreate-test \
-	unicode
+	unicode blank/blank