fix: allow null value in ComboBox validate for single selection (adobe#9742)

Since ComboBox supports allowsCustomValue, the validate function can be
called with an empty selection in single mode. Update ValidationType to
include null for single selection to match the runtime behavior.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: lixiaoyan

packages/@react-types/combobox/src/index.d.ts

@@ -37,7 +37,7 @@ export type MenuTriggerAction = 'focus' | 'input' | 'manual';
 export type SelectionMode = 'single' | 'multiple';
 export type ValueType<M extends SelectionMode> = M extends 'single' ? Key | null : readonly Key[];
 export type ChangeValueType<M extends SelectionMode> = M extends 'single' ? Key | null : Key[];
-type ValidationType<M extends SelectionMode> = M extends 'single' ? Key : Key[];
+type ValidationType<M extends SelectionMode> = M extends 'single' ? Key | null : Key[];
 
 export interface ComboBoxValidationValue<M extends SelectionMode = 'single'> {
   /**