fix incremental label null validation

src/validation/__tests__/DeferStreamDirectiveLabelRule-test.ts

@@ -47,6 +47,19 @@ describe('Validate: Defer/Stream directive labels', () => {
     `);
   });
 
+  it('Defer fragment with null label', () => {
+    expectValid(`
+      {
+        dog {
+          ...dogFragmentA @defer(label: null)
+        }
+      }
+      fragment dogFragmentA on Dog {
+        name
+      }
+    `);
+  });
+
   it('Defer fragment with variable label', () => {
     expectErrors(`
     query($label: String) {
@@ -125,6 +138,16 @@ describe('Validate: Defer/Stream directive labels', () => {
       }
     `);
   });
+  it('Stream with null label', () => {
+    expectValid(`
+      {
+        pets @stream(label: null) {
+          name
+        }
+      }
+    `);
+  });
+
   it('Stream with variable label', () => {
     expectErrors(`
       query ($label: String!) {

src/validation/rules/DeferStreamDirectiveLabelRule.ts

@@ -30,7 +30,7 @@ export function DeferStreamDirectiveLabelRule(
           (arg) => arg.name.value === 'label',
         );
         const labelValue = labelArgument?.value;
-        if (!labelValue) {
+        if (!labelValue || labelValue.kind === Kind.NULL) {
           return;
         }
         if (labelValue.kind !== Kind.STRING) {