Empty css field is not saved to file

Problem/Motivation

When we save an empty CSS field and not disable it, the CSS config is empty, but the file is not regenerated so the old CSS remains even tough the field is empty.

Steps to reproduce

• Enter custom CSS in the field and enable it. Save this
• Remove the custom CSS and save it (do not disable custom CSS)
• Old CSS remains, even tough the field is empty

Proposed resolution

Allow an empty string to be saved to the CSS file.

Remaining tasks

User interface changes

API changes

Data model changes