DataTable sorting by timestamp is alphabetic, not "calendaric"

### Jenkins and plugins versions report

LockableResources-2.18 custom built with table sorting commit added (as of PR #470 https://github.com/jenkinsci/lockable-resources-plugin/pull/470/commits/d07b8c8bb5d2b7ef98080517f7b47789019bed09)
Jenkins LTS 2.387.2

### What Operating System are you using (both controller, and any agents involved in the problem)?

Linux server
Windows browser (Firefox)

### Reproduction steps

Open lockable resources page with several items locked in different months (can be prepared via API `LR.setReservedTimestamp()` call)

Click table header to sort by timestamp

### Expected Results

Sorting is by actual timestamp (date-time) value, ascending or descending as chosen.

Defined (non-null) timestamps are always first.

### Actual Results

Sorting seems to be by months naming (alphabetic).

In descending order, I see Mar-Jan-Apr:

In ascending order, I see lots of empty lines (no timestamp) and then some months.

### Anything else?

_No response_