feat: allow selfNodeId to work on nullable values

In some cases we want a nullable ID, for those the validator should
ignore instead of throwing.

Author: barbieri

lib/selfNodeId.test.ts

@@ -88,6 +88,9 @@ directive @${name} on FIELD_DEFINITION | OBJECT
           type Type3 {
             id: ID!
           }
+          type TypeNullable {
+            id: ID @selfNodeId # test nullable
+          }
           type ${type4} @selfNodeId {
             id: ID!
             anotherField: Float!
@@ -102,6 +105,7 @@ directive @${name} on FIELD_DEFINITION | OBJECT
             type2: ${type2}
             type3: Type3
             type4: ${type4}
+            typeNullable: TypeNullable
             shouldFail: ShouldFail
           }
           type Query {
@@ -128,6 +132,9 @@ directive @${name} on FIELD_DEFINITION | OBJECT
             anotherField
             yetAnotherField
           }
+          typeNullable {
+            id
+          }
           shouldFail {
             float
             array
@@ -155,6 +162,9 @@ directive @${name} on FIELD_DEFINITION | OBJECT
           id: type4Id,
           yetAnotherField: 'asd',
         },
+        typeNullable: {
+          id: null,
+        },
       },
     };
 
@@ -184,6 +194,9 @@ directive @${name} on FIELD_DEFINITION | OBJECT
               id: toNodeId(type4, type4Id),
               yetAnotherField: rootValue.test.type4.yetAnotherField,
             },
+            typeNullable: {
+              id: null,
+            },
           },
         },
         errors: [
@@ -211,6 +224,9 @@ directive @${name} on FIELD_DEFINITION | OBJECT
               id: '2',
             },
             type4: null,
+            typeNullable: {
+              id: null,
+            },
           },
         },
         errors: [

lib/selfNodeId.ts

@@ -21,8 +21,16 @@ export default class SelfNodeIdDirective<
 > extends ValidateDirectiveVisitor<{}, SelfNodeIdContext> {
   public getValidationForArgs(): ValidateFunction<SelfNodeIdContext> {
     const errorMessage = `${this.name} directive only works on strings`;
-    return (value: unknown, _, { name }, { toNodeId }): string => {
+    return (
+      value: unknown,
+      _,
+      { name },
+      { toNodeId },
+    ): string | undefined | null => {
       if (typeof value !== 'string') {
+        if (value === undefined || value === null) {
+          return value;
+        }
         throw new ValidationError(errorMessage);
       }
       const encodedId = toNodeId(name, value);