fix the panic when the list value is nil during tfplan2cai (#15218)

Author: zli82016

mmv1/templates/terraform/expand_property_method.go.tmpl

@@ -88,6 +88,9 @@ func expand{{$.GetPrefix}}{{$.TitlelizeProperty}}(v interface{}, d tpgresource.T
   v = v.(*schema.Set).List()
       {{- end }}
       {{- if or $.NestedProperties (and ($.IsA "NestedObject") $.AllowEmptyObject) }}
+  if v == nil {
+    return nil, nil
+  }
   l := v.([]interface{})
         {{- if $.IsA "Array" }}
   req := make([]interface{}, 0, len(l))