Fix Type Error when null is passed to deepKeys (#26)

* fix: resolve "TypeError: Cannot convert undefined or null to object"

Initially reported in mrodrig/json-2-csv#222
This issue appears when a `null` value is passed for key enumeration because technically `null` is an object, but doesn't have keys. This commit fixes the issue by adding a null check to prevent the type error.

* chore(deps): npm audit fix

* chore(rel): 2.6.0

Author: mrodrig

lib/deeks.js

@@ -15,7 +15,7 @@ module.exports = {
  */
 function deepKeys(object, options) {
     options = mergeOptions(options);
-    if (utils.isObject(object)) {
+    if (utils.isObject(object) && object !== null) {
         return generateDeepKeysList('', object, options);
     }
     return [];

package-lock.json

@@ -1,6 +1,6 @@
 {
   "name": "deeks",
-  "version": "2.5.4",
+  "version": "2.6.0",
   "description": "Retrieve all keys and nested keys from objects and arrays of objects.",
   "main": "lib/deeks.js",
   "scripts": {
@@ -33,7 +33,7 @@
     "@babel/eslint-parser": "7.16.5",
     "coveralls": "3.1.1",
     "eslint": "8.7.0",
-    "mocha": "9.2.0",
+    "mocha": "10.1.0",
     "nyc": "15.1.0",
     "should": "13.2.3"
   },

package.json

@@ -7,6 +7,14 @@ const deeks = require('../lib/deeks.js'),
 describe('deeks Module', () => {
     describe('deepKeys() - Objects', () => {
         describe('Default Options', () => {
+            it('should retrieve no keys for null', (done) => {
+                let keys = deeks.deepKeys(null);
+
+                keys.should.be.an.instanceOf(Array)
+                    .and.have.lengthOf(0);
+                done();
+            });
+
             it('should retrieve no keys for an empty object', (done) => {
                 let testObj = {},
                     keys = deeks.deepKeys(testObj);