assert min_periods is not optional

Author: chelsea-lin

bigframes/core/compile/compiled.py

@@ -458,7 +458,9 @@ def project_window_op(
         if expression.op.skips_nulls and not never_skip_nulls:
             for column in inputs:
                 clauses.append((column.isnull(), ibis_types.null()))
-        if window_spec.min_periods and len(inputs) > 0:
+        
+        assert window_spec.min_periods
+        if len(inputs) > 0:
             if not expression.op.nulls_count_for_min_values:
                 # Most operations do not count NULL values towards min_periods
                 per_col_does_count = (column.notnull() for column in inputs)