fix(jquery-datepicker): clear value when datepicker is cleared

[alebiavati]

Fix bug that prevents user from clearing date value.

Steps to reproduce:

1. Go to https://surveyjs.io/form-library/examples/custom-widget-datepicker/jquery
2. Set date using datepicker
3. Clear date by deleting input text (Delete key)
4. Click Complete button to submit form
5. Click JSON tab to view submitted data

Expected behaviour:

• Date value is empty

Actual behaviour:

• Date value did not clear and is set to:

  { "date": "2024-04-09T01:29:51.000Z" }
  

• JS console error:

  jquery-ui-datepicker.js:120 Uncaught TypeError: Cannot read properties of null (reading 'setHours')
      at r (jquery-ui-datepicker.js:120:13)
      at o.onSelect.o.onSelect (jquery-ui-datepicker.js:101:11)
      at st._selectDate (jquery-ui.min.js:6:101656)
      at st._clearDate (jquery-ui.min.js:6:101467)
      at HTMLInputElement.<anonymous> (jquery-ui-datepicker.js:131:24)
      at HTMLInputElement.dispatch (jquery.min.js:2:43336)
      at y.handle (jquery.min.js:2:41320)
  

Screenshots:

[alebiavati]

Any updates on this?