fix: optional constraints property in ValidationError (#465)

ValidationError.constraint can be undefined if there is only a nested validation error
Close #309

Author: alburkerk

Diff for: src/validation/ValidationError.ts

@@ -25,7 +25,7 @@ export class ValidationError {
     /**
      * Constraints that failed validation with error messages.
      */
-    constraints: {
+    constraints?: {
         [type: string]: string
     };