(fix) Handle empty array return from get_post_taxonomies()

Before: the `::taxonomies()` method was configured to allow for a null
return from `get_post_taxonomies()`. However, it will always return an
array, it's just that the array will be empty if there are no post
taxonomies.

Now: we handle an empty array return.

See: https://developer.wordpress.org/reference/functions/get_post_taxonomies/

spec/page.spec.php

@@ -77,7 +77,7 @@
 		});
 		context('post has no taxonomies', function () {
 			it('returns [\'none\']', function () {
-				allow('get_post_taxonomies')->toBeCalled()->andReturn(null);
+				allow('get_post_taxonomies')->toBeCalled()->andReturn([]);
 				expect($this->page->taxonomies())->toEqual(['none']);
 			});
 		});

src/Page.php

@@ -41,10 +41,7 @@ public function postType(): string
 
 	public function taxonomies(): array
 	{
-		// Note: this implementation is actually buggy
-		// get_post_taxonomies will never return null, only an empty array
-		// Spotted in a refactor, will be fixed in a separate commit
-		return get_post_taxonomies() ?? ['none'];
+		return get_post_taxonomies() ?: ['none'];
 	}
 
 	public function templateName(): string