fix: add undefined initial value to primitive Atom definition

src/vanilla/atom.ts

@@ -26,7 +26,7 @@ type Write<Args extends unknown[], Result> = (
 // This is an internal type and not part of public API.
 // Do not depend on it as it can change without notice.
 type WithInitialValue<Value> = {
-  init: Value
+  init: Value | undefined
 }
 
 type OnUnmount = () => void
@@ -86,14 +86,17 @@ export function atom<Value>(
   initialValue: Value,
 ): PrimitiveAtom<Value> & WithInitialValue<Value>
 
+// primitive atom without default value
+export function atom<Value>(): PrimitiveAtom<Value> & WithInitialValue<Value>
+
 export function atom<Value, Args extends unknown[], Result>(
-  read: Value | Read<Value, SetAtom<Args, Result>>,
+  read?: Value | Read<Value, SetAtom<Args, Result>>,
   write?: Write<Args, Result>,
 ) {
   const key = `atom${++keyCount}`
   const config = {
     toString: () => key,
-  } as WritableAtom<Value, Args, Result> & { init?: Value }
+  } as WritableAtom<Value, Args, Result> & WithInitialValue<Value>
   if (typeof read === 'function') {
     config.read = read as Read<Value, SetAtom<Args, Result>>
   } else {