[BugFix] Fix resolving of image placeholder for Product without images

[naydav]

Steps to reproduce

1. Create Product without images
2. Perform query

{
  products (search: "simple-495") {
    items {
      small_image {
        url
      }
    }
  }
}

Actual result:

 "url": "http://graphql.local/static/version1538424577/graphql/_view/en_US/Magento_Catalog/images/product/placeholder/small_image.jpg"

Expected result:

 "url": "http://graphql.local/static/version1538424577/frontend/Magento/luma/en_US/Magento_Catalog/images/product/placeholder/small_image.jpg"

Problem is related to wrong area
Also, need to finish \Magento\GraphQl\Catalog\ProductImageTest::testProductWithoutBaseImage

[naydav]

#364

[bchatard]

@naydav I test the patch in the PR #364 and the issue is still present.

The Magento\CatalogGraphQl\Model\Resolver\Products\DataProvider\Image\Placeholder\Theme class is no more used in the merged version, while it is this class that seemed to make the correction