Assets field: "Restrict allowed file types" without checking options

[janhenckens]

Description

When editing an assets field, it's now possible to check the "Restrict allowed file types?" checkbox and save the field without specifying any file types to which it should be restricted. That results in a field that doesn't accept any file types.

Simple mistake from the users part but could we make at least one of the file type checkboxes required if restrictFiles gets checked?

Additional info

• Craft version: 3.0.28
• PHP version: 7.2.1
• Database driver & version: 5.6.38
• Plugins & versions: None