Allow for optional explanation.

Business Link method accepts None, so allow to call global build on Business
Process itself.

product/ERP5/Document/BusinessProcess.py

@@ -835,7 +835,7 @@ class BusinessProcess(Path, XMLObject):
     return not not self.getPartiallyBuildableBusinessLinkValueList(explanation)
 
   security.declareProtected(Permissions.AccessContentsInformation, 'build')
-  def build(self, explanation):
+  def build(self, explanation=None):
     """
       Build whatever is buildable
     """