Accept unexisting members in dues list

* Closes: #32

src/dues/DuesReport.js

 import React, { useState } from "react";
 import { Table } from "react-bootstrap";
 import Fetcher from "../Fetcher";
-import { printDate, printMoney } from "../util";
+import { printDate, printMoney, printMember } from "../util";
 
 function DuesReport() {
   const [dues, setDues] = useState([]);
@@ -18,7 +18,7 @@ function DuesReport() {
     const entries = dues.map((d) => (
       <tr key={d.ID}>
         <td>{printDate(d.date)}</td>
-        <td>{d.member.name + " (" + d.member.num + ")"}</td>
+        <td>{printMember(d.member)}</td>
         <td>{printMoney(-d.total)}</td>
       </tr>
     ));

src/util.js

@@ -38,6 +38,14 @@ function printID(pre, item) {
   return pre + "-" + item.ID.toString().padStart(5, "0");
 }
 
+function printMember(member) {
+  if (member) {
+    return member.name + " (" + member.num + ")";
+  } else {
+    return "-";
+  }
+}
+
 function url(path) {
   let api = process.env.REACT_APP_API;
   if (!api) {
@@ -75,6 +83,7 @@ export {
   printMoney,
   printDate,
   printRole,
+  printMember,
   url,
   date2string,
   time2string,