fix(mir-semantics): Handle zero sized function constant decode

Author: Stevengre

kmir/src/kmir/kdist/mir-semantics/rt/data.md

@@ -1468,6 +1468,9 @@ Zero-sized types can be decoded trivially into their respective representation.
   // zero-sized array
   rule <k> #decodeConstant(constantKindZeroSized, _TY, typeInfoArrayType(_, _))
         => Range(.List) ... </k>
+  // zero-sized function item (e.g., closures without captures)
+  rule <k> #decodeConstant(constantKindZeroSized, _TY, typeInfoFunType(_))
+        => Aggregate(variantIdx(0), .List) ... </k>
 ```
 
 Allocated constants of reference type with a single provenance map entry are decoded as references