Use empty string for empty descriptions to construct grouping key

Closes #12952

modules/project_invoice/project.py

@@ -577,7 +577,7 @@ class Work(Effort, Progress, Timesheet, metaclass=PoolMeta):
         return (('product', line['product']),
             ('unit', line['unit']),
             ('unit_price', line['unit_price']),
-            ('description', line['description']))
+            ('description', line['description'] or ''))
 
     def _get_invoice_line(self, key, invoice, lines):
         "Return a invoice line for the lines"

modules/purchase_request/purchase_request.py

@@ -391,7 +391,7 @@ class CreatePurchase(Wizard):
         '''
         return (
             ('product', request.product),
-            ('description', request.description),
+            ('description', request.description or ''),
             ('unit', request.unit),
             )
 

modules/sale_subscription/subscription.py

@@ -957,7 +957,7 @@ class LineConsumption(ModelSQL, ModelView):
             ('unit', consumption.line.unit),
             ('product', consumption.line.service.product),
             ('unit_price', consumption.line.unit_price),
-            ('description', consumption.line.description),
+            ('description', consumption.line.description or ''),
             ('origin', consumption.line),
             )