Node null check fix

smithy-model/src/main/java/software/amazon/smithy/model/knowledge/NullableIndex.java

@@ -179,8 +179,9 @@ public boolean isMemberNullable(MemberShape member, CheckMode checkMode) {
                 }
                 // fall-through.
             case LIST:
-                // Map values and list members are only null if they have the @sparse trait.
-                return container.hasTrait(SparseTrait.ID);
+                // Map values and list members are only null if they have the @sparse trait
+                // OR if the target is a Document since Document can hold a null value
+                return container.hasTrait(SparseTrait.ID) || target.isDocumentShape();
             default:
                 return false;
         }

smithy-model/src/test/java/software/amazon/smithy/model/validation/NodeValidationVisitorTest.java

@@ -675,6 +675,24 @@ public void nullNonSparseMapValue() {
                         "a: Non-sparse map shape `ns.foo#Map` cannot contain null values"));
     }
 
+    @Test
+    public void nullNonSparseDocumentMapValue() {
+        // This should not raise any errors since null is a valid Document value
+        ObjectNode map = ObjectNode.builder()
+                .withMember("a", Node.nullNode())
+                .build();
+        NodeValidationVisitor visitor = NodeValidationVisitor.builder()
+                .value(map)
+                .model(MODEL)
+                .allowOptionalNull(true)
+                .build();
+        List<ValidationEvent> events = MODEL
+                .expectShape(ShapeId.from("ns.foo#DocumentMap"))
+                .accept(visitor);
+
+        assertThat(events, hasSize(0));
+    }
+
     @Test
     public void nullSparseMapValue() {
         ObjectNode map = ObjectNode.builder()

smithy-model/src/test/resources/software/amazon/smithy/model/validation/node-validator.json

@@ -207,6 +207,15 @@
                 "smithy.api#sparse": {}
             }
         },
+        "ns.foo#DocumentMap": {
+            "type": "map",
+            "key": {
+                "target": "ns.foo#KeyString"
+            },
+            "value": {
+                "target": "smithy.api#Document"
+            }
+        },
         "ns.foo#Structure": {
             "type": "structure",
             "members": {