BUGFIX: Initialize `constraints` in `read_castep_castep`

Closes #1524 (closed)

This MR solves the issue above by initializing constraints in read_castep_castep.

Since the change is minor, I would like to merge this soon.

Checklist

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.
• Issue is resolved via "closes #XXXX" if applicable.