add expect().toBeNull() (#47716)

Summary:
Pull Request resolved: #47716

Add toBeNull check to Fantom.

Jest docs: https://jestjs.io/docs/expect#tobenull

Differential Revision: D66167811

Author: sammy-SC

jest/integration/runtime/setup.js

@@ -224,6 +224,15 @@ class Expect {
     }
   }
 
+  toBeNull(): void {
+    const pass = this.#received == null;
+    if (!this.#isExpectedResult(pass)) {
+      throw new Error(
+        `Expected ${String(this.#received)}${this.#maybeNotLabel()} to be null`,
+      );
+    }
+  }
+
   toThrow(expected?: string): void {
     if (expected != null && typeof expected !== 'string') {
       throw new Error(