Allow vars to be missing when def is nullable

Allow variables to be missing when the associated variable definition is
nullable (in which case we use `null` as the default).

This fixes queries like

```graphql
query Test($var: Int) {
    test(var: $var)
}
```

```json
{}
```

which appear to be allowed according to the GraphQL spec.

src/context.rs

@@ -424,6 +424,16 @@ impl<'a, T> ContextBase<'a, T> {
             } else if let Some(default) = &def.default_value {
                 return Ok(default.clone_inner());
             }
+            match def.var_type.deref() {
+                &async_graphql_parser::query::Type::Named(_)
+                | &async_graphql_parser::query::Type::List(_) => {
+                    // Nullable types can default to null when not given.
+                    return Ok(Value::Null);
+                }
+                &async_graphql_parser::query::Type::NonNull(_) => {
+                    // Strict types can not.
+                }
+            }
         }
         Err(QueryError::VarNotDefined {
             var_name: name.to_string(),

tests/variables.rs

@@ -71,6 +71,69 @@ pub async fn test_variable_default_value() {
     );
 }
 
+#[async_std::test]
+pub async fn test_variable_no_value() {
+    struct QueryRoot;
+
+    #[Object]
+    impl QueryRoot {
+        pub async fn int_val(&self, value: Option<i32>) -> i32 {
+            value.unwrap_or(10)
+        }
+    }
+
+    let schema = Schema::new(QueryRoot, EmptyMutation, EmptySubscription);
+    let query = QueryBuilder::new(
+        r#"
+            query QueryWithVariables($intVal: Int) {
+                intVal(value: $intVal)
+            }
+        "#,
+    )
+    .variables(Variables::parse_from_json(serde_json::json!({})).unwrap());
+    let resp = query.execute(&schema).await.unwrap();
+    assert_eq!(
+        resp.data,
+        serde_json::json!({
+            "intVal": 10,
+        })
+    );
+}
+
+#[async_std::test]
+pub async fn test_variable_null() {
+    struct QueryRoot;
+
+    #[Object]
+    impl QueryRoot {
+        pub async fn int_val(&self, value: Option<i32>) -> i32 {
+            value.unwrap_or(10)
+        }
+    }
+
+    let schema = Schema::new(QueryRoot, EmptyMutation, EmptySubscription);
+    let query = QueryBuilder::new(
+        r#"
+            query QueryWithVariables($intVal: Int) {
+                intVal(value: $intVal)
+            }
+        "#,
+    )
+    .variables(
+        Variables::parse_from_json(serde_json::json!({
+            "intVal": null,
+        }))
+        .unwrap(),
+    );
+    let resp = query.execute(&schema).await.unwrap();
+    assert_eq!(
+        resp.data,
+        serde_json::json!({
+            "intVal": 10,
+        })
+    );
+}
+
 #[async_std::test]
 pub async fn test_variable_in_input_object() {
     #[InputObject]