Ensure null is not treated as an object during validation (#4630)

Author: lazerg

lib/is.mjs

@@ -13,7 +13,7 @@ const defined = (val) => typeof val !== 'undefined' && val !== null;
  * Is this value an object?
  * @private
  */
-const object = (val) => typeof val === 'object';
+const object = (val) => typeof val === 'object' && val !== null;
 
 /**
  * Is this value a plain object?

test/unit/composite.js

@@ -371,6 +371,13 @@ suite('composite', () => {
       }, /Expected object for image to composite but received invalid of type string/);
     });
 
+    test('null image', (t) => {
+      t.plan(1);
+      t.assert.throws(() => {
+        sharp().composite([null]);
+      }, /Expected object for image to composite but received null of type object/);
+    });
+
     test('missing input', (t) => {
       t.plan(1);
       t.assert.throws(() => {

test/unit/extend.js

@@ -155,6 +155,13 @@ suite('Extend', () => {
       /Expected integer between 1 and 100000000 for extend but received -1 of type number/
     );
   });
+  test('null fails', (t) => {
+    t.plan(1);
+    t.assert.throws(
+      () => sharp().extend(null),
+      /Expected integer or object for extend but received null of type object/
+    );
+  });
   test('too large fails', (t) => {
     t.plan(1);
     t.assert.throws(