Set default CONST term in Expr children

Initializes Expr children with {CONST: 0.0} by default instead of an empty dict. This ensures that expressions always have a constant term, which may help avoid issues with missing constants in further computations.

Author: Zeroto521

src/pyscipopt/expr.pxi

@@ -62,7 +62,7 @@ class Expr:
     """Base class for mathematical expressions."""
 
     def __init__(self, children: Optional[dict[Union[Variable, Term, Expr], float]] = None):
-        children = children or {}
+        children = children or {CONST: 0.0}
         if not all(isinstance(i, (Variable, Term, Expr)) for i in children):
             raise TypeError("All keys must be Variable, Term or Expr instances")