Pass Expected Data Type in backgroundColor Call (2.3)

[northernco]

Description

\Magento\Framework\Image::backgroundColor is expecting null|array as valid parameter types. However, in \Magento\Swatches\Helper\Media::setupImageProperties the string value '#FFF' is passed into this function call. The goal of this fix is to pass along the correct and expected type, which in this case is going to be an array of RGB values to produce the same end result.

This can also be seen on 2.1-develop (addressed in #13916) and 2.2-develop (addressed in #13914).

Fixed Issues (if relevant)

Manual testing scenarios

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)

Forwardport for #13914

[sidolov]

Fixed unit test according to changes