Fixed issue with not correct updating products category_ids through /rest/ for custom_attribute { 'category_ids } data.

Description (*)

Added call of assignProductToCategories method in \Magento\Catalog\Model\ProductRepository::save
in case when a product currently don't have extension attributes.
This is needed to fix a problem with not correct updating category_ids when try to update the product with /rest/all/V1/products/ through custom_attribute { 'category_ids': '['10127', '10048'] },

Fixed Issues

#20481: REST products update category_ids cannot be removed

Manual testing scenarios (*)

1. Open in swagger catalogProductRepositoryV1 and call PUT: /V1/products/{sku}

2. Use test product with such test case:
   "custom_attributes": [
   {
   "attribute_code": "category_ids",
   "value": [
   "1", "2", "3", "4"
   ]
   }
   ]
   

Result will be such: all category_ids with provided id's "1", "2", "3", "4" will be assigned to product.

3. Then try to remove some categories:
   "custom_attributes": [
   {
   "attribute_code": "category_ids",
   "value": [
   "1", "3"
   ]
   }
   ]
   Result will be such: category_ids with provided id's "1", "3" will be assigned to product.
   Other category_ids, that where before, will be unassigned from product, as expected.

So now category_ids are updated correct!

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)