forms: fix free length table error check to ignore its emptiness (#39072)

tests/test_form_pages.py

@@ -2972,6 +2972,17 @@ def test_form_new_table_rows_field_draft_recall(pub):
     assert formdef.data_class().select()[0].data['1'] == 'test'
     assert formdef.data_class().select()[0].data['3'] is None
 
+def test_form_table_rows_field_and_prefill(pub, emails):
+    formdef = create_formdef()
+    formdef.fields = [
+        fields.TableRowsField(id='0', label='table', type='tablerows', columns=['a', 'b'], required=True),
+        fields.StringField(id='1', label='string', prefill={'type': 'string', 'value': 'HELLO WORLD'}),
+    ]
+    formdef.store()
+    formdef.data_class().wipe()
+
+    resp = get_app(pub).get('/test/')
+
 def test_formdata_attachment_download(pub):
     create_user(pub)
     wf = Workflow(name='status')

wcs/qommon/form.py

@@ -1741,7 +1741,7 @@ class TableListRowsWidget(WidgetListAsTable):
         if (request.form or request.get_method() == 'POST') and self.required:
             if not self.value and not add_element_pushed:
                 self.set_error(self.REQUIRED_ERROR)
-            for row in self.value:
+            for row in self.value or []:
                 for column in row:
                     if column:
                         break