return false when checking if undefined or null

spiffworkflow-frontend/src/components/ProcessInstanceListTable.tsx

@@ -1698,8 +1698,8 @@ export default function ProcessInstanceListTable({
   };
 
   const formatDateTime = (_row: ProcessInstance, value: any) => {
-    if (value === undefined) {
-      return undefined;
+    if (value === undefined || value === null) {
+      return value;
     }
     let dateInSeconds = value;
     if (!isANumber(value)) {

spiffworkflow-frontend/src/helpers.tsx

@@ -353,7 +353,10 @@ export const setPageTitle = (items: Array<string>) => {
 
 // calling it isANumber to avoid confusion with other libraries
 // that have isNumber methods
-export const isANumber = (str: string | number) => {
+export const isANumber = (str: string | number | null) => {
+  if (str === undefined || str === null) {
+    return false;
+  }
   return /^\d+(\.\d+)?$/.test(str.toString());
 };