Fix a segmentation fault with cleanup_var

[user202729]

fix #41127

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[cxzhong]

I tested it is fixed

sage: SR.cleanup_var('x')
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[1], line 1
----> 1 SR.cleanup_var('x')

File sage/symbolic/ring.pyx:738, in sage.symbolic.ring.SymbolicRing.cleanup_var()

File sage/symbolic/ring.pyx:937, in sage.symbolic.ring.SymbolicRing._repr_element_()

File sage/symbolic/expression.pyx:13534, in sage.symbolic.expression._repr_Expression()

TypeError: Cannot convert str to sage.symbolic.expression.Expression