fix(helpers): keep falsy but defined values in fillTemplate

fillTemplate replaced each ${var} with `templateVars[match] || ''`,
which also dropped values that are defined but falsy, such as the
number 0 or the boolean false. For example fillTemplate('${n}', {n: 0})
returned an empty string instead of '0'.

This is noticeable in Pagination, where PaginationOptionsMenu passes
numeric firstIndex, lastIndex and itemCount into a string toggle
template. With an empty data set (itemCount of 0) the '0' was silently
dropped from the toggle text.

Use the nullish coalescing operator so only null or undefined values
(for example a missing key) fall back to an empty string, while 0 and
false are kept.

Co-authored-by: eeshsaxena <eeshsaxena@gmail.com>

Author: Arunendra21

packages/react-core/src/helpers/__tests__/util.test.ts

@@ -99,6 +99,16 @@ test('fillTemplate interpolates strings correctly', () => {
   expect(actual).toEqual(expected);
 });
 
+test('fillTemplate keeps falsy but defined values such as 0 and false', () => {
+  expect(fillTemplate('count: ${n}', { n: 0 })).toEqual('count: 0');
+  expect(fillTemplate('value: ${v}', { v: false })).toEqual('value: false');
+  expect(fillTemplate('empty:${e}!', { e: '' })).toEqual('empty:!');
+});
+
+test('fillTemplate replaces missing keys with an empty string', () => {
+  expect(fillTemplate('x=${missing}', {})).toEqual('x=');
+});
+
 test('text pluralize', () => {
   expect(pluralize(1, 'dog')).toEqual('1 dog');
   expect(pluralize(2, 'dog')).toEqual('2 dogs');

packages/react-core/src/helpers/util.ts

@@ -114,7 +114,7 @@ export function sideElementIsOutOfView(container: HTMLElement, element: HTMLElem
  * @returns {string} The template string literal result
  */
 export function fillTemplate(templateString: string, templateVars: any) {
-  return templateString.replace(/\${(.*?)}/g, (_, match) => templateVars[match] || '');
+  return templateString.replace(/\${(.*?)}/g, (_, match) => templateVars[match] ?? '');
 }
 
 /**