Correct typing error in SyncedRef.current property

This issue was not caught previously because the property is only written by JS
code in the `preact` library.

Author: robertknight

src/hooks/use-synced-ref.ts

@@ -23,7 +23,7 @@ class SyncedRef<T> implements RefObject<T> {
     return this._value;
   }
 
-  set current(value: null) {
+  set current(value: T | null) {
     this._value = value;
     this._updateTarget();
   }