fix: null string to bytes conversions (#5492)

Avoid a panic if the bytes conversion function receives a string
typed NULL value. The function now returns the same error as for
other NULL values.

Author: mhilton

stdlib/universe/typeconv.go

@@ -374,6 +374,9 @@ var byteConv = values.NewFunction(
 		} else if v.Type().Nature() == semantic.Dynamic {
 			v = v.Dynamic().Inner()
 		}
+		if v.IsNull() {
+			v = values.Null
+		}
 
 		switch v.Type().Nature() {
 		case semantic.String:

stdlib/universe/typeconv_test.go

@@ -10,6 +10,7 @@ import (
 	"github.com/influxdata/flux/dependencies/dependenciestest"
 	"github.com/influxdata/flux/dependency"
 	"github.com/influxdata/flux/memory"
+	"github.com/influxdata/flux/semantic"
 	"github.com/influxdata/flux/values"
 )
 
@@ -786,3 +787,17 @@ func TestTypeconv_Duration(t *testing.T) {
 		})
 	}
 }
+
+func TestTypeconv_Bytes_NullString(t *testing.T) {
+	myMap := map[string]values.Value{
+		"v": values.NewNull(semantic.BasicString),
+	}
+	args := values.NewObjectWithValues(myMap)
+	c := byteConv
+	ctx, deps := dependency.Inject(context.Background(), dependenciestest.Default())
+	defer deps.Finish()
+	_, err := c.Call(ctx, args)
+	if err == nil || err.Error() != "cannot convert null to bytes" {
+		t.Errorf(`Expected error "cannot convert null to bytes", got %q`, err)
+	}
+}