treat return value of write_<par> correctly

if the return value is 0 or "", and value is different from it,
then the behaviour is incorrect.
Alternative: we do not allow write_<par> to return None, and
check for it, or even validate the returned value.

Change-Id: If7381dd06f7c55bdc4a80981e67f831cd8ee4136
Reviewed-on: https://forge.frm2.tum.de/review/19728
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>

secop/metaclass.py

@@ -170,7 +170,9 @@ class ModuleMeta(type):
                     value = pobj.datatype.validate(value)
                     if wfunc:
                         self.log.debug('calling %r(%r)' % (wfunc, value))
-                        value = wfunc(self, value) or value
+                        returned_value = wfunc(self, value)
+                        if returned_value is not None:
+                            value = returned_value
                     # XXX: use setattr or direct manipulation
                     # of self.accessibles[pname]?
                     setattr(self, pname, value)