vam: Fix casting on const null (#5471)

Author: mattnibs

runtime/vam/expr/cast/cast.go

@@ -72,6 +72,9 @@ func assemble(zctx *super.Context, vec vector.Any, typ super.Type, fn caster) ve
 }
 
 func castConst(zctx *super.Context, vec *vector.Const, typ super.Type) vector.Any {
+	if vec.Type().ID() == super.IDNull {
+		return vector.NewConst(super.NewValue(typ, nil), vec.Len(), nil)
+	}
 	val := samexpr.LookupPrimitiveCaster(zctx, typ).Eval(samexpr.NewContext(), vec.Value())
 	if val.IsError() {
 		if vec.Nulls != nil {

runtime/ztests/expr/cast/nulls.yaml

@@ -0,0 +1,19 @@
+zed: |
+  yield uint64(this), int64(this), float64(this), bool(this), bytes(this),
+        string(this), ip(this), net(this), type(null)
+
+vector: true
+
+input: |
+  null
+
+output: |
+  null(uint64)
+  null(int64)
+  null(float64)
+  null(bool)
+  null(bytes)
+  null(string)
+  null(ip)
+  null(net)
+  null(type)