still eval even if optional + null (#763)

Author: mr-c

cwltool/builder.py

@@ -324,8 +324,6 @@ def do_eval(self, ex, context=None, pull_image=True, recursive=False, strip_whit
                 return {k: self.do_eval(v, context, pull_image, recursive) for k, v in iteritems(ex)}
             if isinstance(ex, list):
                 return [self.do_eval(v, context, pull_image, recursive) for v in ex]
-        if context is None and type(ex) is str and "self" in ex:
-            return None
         return expression.do_eval(ex, self.job, self.requirements,
                                   self.outdir, self.tmpdir,
                                   self.resources,