Make BlockToolbar work with empty configuration.

[jodator]

Suggested merge commit message (convention)

Fix: Make BlockToolbar work with an empty configuration. Closes ckeditor/ckeditor5#5980.

---

Additional information

• As discussed in config may not exist #532 the block toolbar should work with empty toolbar configuration as other features use this pattern.

[Reinmar]

I checked out src/ to master and expected this test to fail, but it didn't. The other one did, but then why didn't this one? Maybe expect() doesn't handle async functions?

[jodator]

I'll check it then.

[jodator]

Oh... that was not wise for me 😥.

With promises we have three choices:

1. Expecting to not fail not explicitly (anyway test must pass)

   editor = await ClassicTestEditor.create( element, {
     plugins: [ BlockToolbar ]
   } );
   // Optional comment: The test must not throw or rename the test
   //  to "should not throw when empty config is provided"

2. Explicitly fail on error

   try {
   	editor = await ClassicTestEditor.create( element, {
   		plugins: [ BlockToolbar ]
   	} );
   } catch ( error ) {
   	expect.fail( 'Error should not be thrown.' );
   }

3. Use chai-as-promised:

   expect( async () => {
   	editor = await ClassicTestEditor.create( element, {
   		plugins: [ BlockToolbar ]
   	} );
   } ).to.evantually.not.throw();

I'm for 1. But every other way works (3 needs update in the dev-tests to add chai-as-promised.

[Reinmar]

1. is fine for me too.

[Reinmar]

The code is good but I'd like to clarify why one of the tests doesn't fail on master.