Add an empty_value_display property on Column for use when value is blank

This surfaces an issue with SearchPromotion having a broken string representation when the query string is None, so fix that too

wagtail/admin/ui/tables/__init__.py

@@ -141,6 +141,7 @@ class Column(BaseColumn):
     """A column that displays a single field of data from the model"""
 
     cell_template_name = "wagtailadmin/tables/cell.html"
+    empty_value_display = ""
 
     def get_value(self, instance):
         """
@@ -163,7 +164,11 @@ class Column(BaseColumn):
             # on templates to be explicitly localized or unlocalized. For numeric table cells, we
             # unlocalize them by default; developers may subclass Column to obtain formatted numbers.
             value = unlocalize(value)
-        context["value"] = value
+
+        if not str(value).strip():
+            context["value"] = self.empty_value_display
+        else:
+            context["value"] = value
         return context
 
 
@@ -185,6 +190,7 @@ class TitleColumn(Column):
     """A column where data is styled as a title and wrapped in a link or <label>"""
 
     cell_template_name = "wagtailadmin/tables/title_cell.html"
+    empty_value_display = gettext_lazy("(blank)")
 
     def __init__(
         self,
@@ -211,8 +217,6 @@ class TitleColumn(Column):
 
     def get_cell_context_data(self, instance, parent_context):
         context = super().get_cell_context_data(instance, parent_context)
-        if not str(context["value"]).strip():
-            context["value"] = "(blank)"
         context["link_attrs"] = self.get_link_attrs(instance, parent_context)
         context["link_attrs"]["href"] = context["link_url"] = self.get_link_url(
             instance, parent_context

wagtail/contrib/search_promotions/models.py

@@ -144,13 +144,7 @@ class SearchPromotion(models.Model):
         else:
             label = "external link"
 
-        return (
-            'SearchPromotion(query="'
-            + self.query.query_string
-            + f'", {label}="'
-            + self.title
-            + '")'
-        )
+        return f'SearchPromotion(query="{self.query.query_string}", {label}="{self.title}")'
 
     def __str__(self):
         return f"{self.query.query_string} - {self.title}"