Don't use srcset if srcset is about:blank

Problem/Motivation

Due to lazy loading the srcset attribute is set, but it's set to about:blank.
This means the correct srcset in data-srcset is never processed.

Steps to reproduce

I'm not sure of the module or code responsible for setting the about:blank attribute.

Proposed resolution

If srcset attribute is present, but srcset is set to about:blank, use the data-srcset instead.

The patch attached solve the issue I'm facing.
I'm sharing here in case anyone else faces the same issue.