entc/gen: set foreign-key columns non-nullable for required edges (#1703)

Note, this only applies to edges without circular references.
Fixed #1688 and #1374

dialect/sql/schema/schema.go

@@ -409,9 +409,6 @@ const (
 
 // ConstName returns the constant name of a reference option. It's used by entc for printing the constant name in templates.
 func (r ReferenceOption) ConstName() string {
-	if r == NoAction {
-		return ""
-	}
 	return strings.ReplaceAll(strings.Title(strings.ToLower(string(r))), " ", "")
 }