test: cover string user ids in workbench payloads

internal/httpapi/uid.go

@@ -27,8 +27,6 @@ type optionalUserID struct {
 
 func (id *optionalUserID) UnmarshalJSON(raw []byte) error {
 	if strings.TrimSpace(string(raw)) == "null" {
-		id.set = true
-		id.value = 0
 		return nil
 	}
 	value, err := parseUserIDJSON(raw)
@@ -52,11 +50,7 @@ func parseUserIDJSON(raw []byte) (int64, error) {
 	if err := json.Unmarshal(raw, &text); err == nil {
 		return parseUserID(text)
 	}
-	var number int64
-	if err := json.Unmarshal(raw, &number); err == nil {
-		return number, nil
-	}
-	return 0, fmt.Errorf("invalid user id")
+	return 0, fmt.Errorf("user id must be a string")
 }
 
 func parseUserID(value string) (int64, error) {