gui: tidy up workbench menu

constraint.py

@@ -372,7 +372,7 @@ def _a(solver,partInfo,subname,shape,retAll=False):
 
 
 class ConstraintCommand:
-    _menuGroupName = ''
+    _menuGroupName = None
 
     def __init__(self,tp):
         self.tp = tp

gui.py

@@ -404,6 +404,7 @@ class AsmCmdAddWorkplane(AsmCmdBase):
     _menuText = 'Add workplane'
     _iconName = 'Assembly_Add_Workplane.svg'
     _toolbarName = None
+    _menuGroupName = None
     _accel = 'A, P'
     _makeType = 0
 
@@ -449,6 +450,7 @@ class AsmCmdAddOrigin(AsmCmdAddWorkplane):
 
 class AsmCmdAddWorkplaneGroup(AsmCmdAddWorkplane):
     _id = 12
+    _menuGroupName = ''
     _toolbarName = AsmCmdBase._toolbarName
     _cmds = (AsmCmdAddWorkplane.getName(),
              AsmCmdAddWorkplaneXZ.getName(),