[11.x] Add tests for handling non-baked enum and empty string requests

[hrant1020]

        /** Added some tests */
        $request = Request::create('/', 'GET', [
            'empty_value_request' => '',
        ]);

        // Empty string case         
        $this->assertNull($request->enum('empty_value_request', TestEnumBacked::class)); //this should be null
        
        // Non baked enum case         
        $this->assertNull($request->enum('valid_enum_value', TestEnum::class)); //this should be null (no error thrown)       

So now PR test look like this

    public function testEnumMethod()
    {
        $request = Request::create('/', 'GET', [
            'valid_enum_value' => 'test',
            'invalid_enum_value' => 'invalid',
            'empty_value_request' => '',
        ]);

        $this->assertNull($request->enum('doesnt_exists', TestEnumBacked::class));

        $this->assertEquals(TestEnumBacked::test, $request->enum('valid_enum_value', TestEnumBacked::class));

        $this->assertNull($request->enum('invalid_enum_value', TestEnumBacked::class));
        $this->assertNull($request->enum('empty_value_request', TestEnumBacked::class));
        $this->assertNull($request->enum('valid_enum_value', TestEnum::class));
    }