Fix `test_blank_columns_created_in_block`

Follow up to 1683410c.

activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb

@@ -225,7 +225,7 @@ def columns(table_name) # :nodoc:
             type_metadata = fetch_type_metadata(column_name, type, oid, fmod)
             default_value = extract_value_from_default(default)
             default_function = extract_default_function(default_value, default)
-            new_column(column_name, default_value, type_metadata, !notnull, table_name, default_function, collation, comment: comment)
+            new_column(column_name, default_value, type_metadata, !notnull, table_name, default_function, collation, comment: comment.presence)
           end
         end