fix(isNotNil): simplify isNotNil and add testcases (#519)

* Simplify isNotNil and add testcases

* fix chore

src/predicate/isBoolean.spec.ts

@@ -1,7 +1,7 @@
 import { describe, expect, it } from 'vitest';
 import { isBoolean } from './isBoolean';
 
-describe('isNull', () => {
+describe('isBoolean', () => {
   it('returns true if the value is boolean', () => {
     expect(isBoolean(true)).toBe(true);
     expect(isBoolean(false)).toBe(true);

src/predicate/isNotNil.spec.ts

@@ -10,6 +10,10 @@ describe('isNotNil', () => {
   it('returns true if the value is not null nor undefined', () => {
     expect(isNotNil('')).toBe(true);
     expect(isNotNil(123)).toBe(true);
+    expect(isNotNil(0)).toBe(true);
+    expect(isNotNil(false)).toBe(true);
+    expect(isNotNil(true)).toBe(true);
+    expect(isNotNil([])).toBe(true);
   });
 
   it('can be used with TypeScript as a type predicate', () => {

src/predicate/isNotNil.ts

@@ -15,5 +15,5 @@
  * // result will be [1, 3]
  */
 export function isNotNil<T>(x: T | null | undefined): x is T {
-  return x !== null && x !== undefined;
+  return x != null;
 }