Missing check for empty field when building ctools block

Problem/Motivation

Some background: I have placed a ctools block, for a field based on a custom field type, which works almost as expected. The custom field type is extended from the text_long field type in Core. Ordinarily, if the field is empty, then the block will not render. My understanding is that is the correct and desired behavior in general. I have not been able to successfully reproduce the below problem outside of this instance of my custom field type:

What's happening is that when the field is empty, despite the isEmpty() method, as defined in the field type definition, returning TRUE, the ctools block provides an empty field when building the block, which results in a block rendered without any content in it. The expectation is that the block wouldn't render at all since the field is empty. For the purposes of block rendering, an element is considered empty by the renderer if the element is either empty, or only contains the #cache index. The build() method in EntityField.php adds a field index to the element without checking whether the field is empty or not.

Proposed resolution

Add a conditional to check if the field is empty, before adding the field index to the $build variable, that way the renderer will consider the element empty if the field is empty, and continue to render content as usual if the field is not empty.

Remaining tasks

Needs review.

User interface changes

None.

API changes

None.

Data model changes

None