Describe the bug
The Excerpt Length tool in Design > Blog > Blog Page > Post List Settings indicates that it allows the user to set an excerpt length of 0, but the minimum allowed excerpt length appears to be 1. After saving the field value is automatically changed to 1.

To Reproduce
Steps to reproduce the behavior:

1. Go to Design > Blog > Blog Page > Post List Settings
2. Change Excerpt Length to 0
3. Preview should show the first word of the post content instead of just the Read More button as expected
4. Publish changes
5. Reload the controls
6. See that Excerpt Length is set to 1 instead of the expected 0

Screenshots