fix(front): constructing flaky test branches list (#75)

## Description
Constructing a list of branches in flaky test notifications breaks when
no branches are specified.

## Type of Change
<!-- Mark relevant items with an [x] -->
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation/Repository docs update
- [ ] Performance improvement
- [ ] Code refactoring
- [ ] Test updates

## Testing
<!-- How has this been tested? -->
- [ ] Unit tests added/updated
- [ ] Integration tests added/updated
- [x] Manual testing performed
- [ ] Not applicable

## Documentation
<!-- Mark relevant items with an [x] -->
- [ ] Documentation update required
- [ ] Changelog update required

Author: skipi

front/assets/js/flaky_tests/components/notification.tsx

@@ -233,6 +233,8 @@ export const Notification = (props: NotificationProps) => {
     }
   };
 
+  const branches = settings?.branches?.join(`, `);
+
   return (
     <Fragment>
       <div>
@@ -310,7 +312,7 @@ export const Notification = (props: NotificationProps) => {
             type="text"
             className="form-control w-100"
             placeholder="e.g. master,prod-*,.*"
-            value={settings?.branches.join(` `)}
+            value={branches}
             onInput={onBranchesChange}
           />
           <p className="f6 mt1 mb0 nb1">