empty strings in target should be skipped

inst/private/doctest_collect.m

@@ -19,6 +19,10 @@
 % TODO: methods('logical') octave/matlab differ: which behaviour do we want?
 % TODO: what about builtin "test" versus dir "test/"?  Do we prefer dir?
 
+if (isempty(w))
+  return
+end
+
 % determine type of target
 if is_octave()
   % Note: ripe for refactoring once "exist(w, 'class')" works in Octave.

test/bist.m

@@ -134,3 +134,20 @@ function bist()
 %! assert (n == 0)
 %! assert (t == 0)
 %! assert (summ.num_targets == 0)
+
+%!test
+%! % skip empty targets
+%! [n, t, summ] = doctest({'', ''});
+%! assert (n == 0)
+%! assert (t == 0)
+%! assert (summ.num_targets == 0)
+%! assert (summ.num_targets_with_extraction_errors == 0)
+
+%!test
+%! % skip empty targets
+%! [n1, t1, summ1] = doctest('doctest');
+%! [n2, t2, summ2] = doctest({'', '', 'doctest', ''});
+%! assert (n1 == n2)
+%! assert (t1 == t2)
+%! assert (summ1.num_targets == summ2.num_targets)
+%! assert (summ2.num_targets_with_extraction_errors == 0)