TypeError: Cannot access offset of type string on string in Drupal\views\Plugin\views\filter\NumericFilter->valueForm()

Problem/Motivation

TypeError: Cannot access offset of type string on string in Drupal\views\Plugin\views\filter\NumericFilter->valueForm() (line 239 of core/modules/views/src/Plugin/views/filter/NumericFilter.php).

Drupal\views\Plugin\views\filter\NumericFilter->valueForm(Array, Object) (Line: 40)
Drupal\views\Plugin\views\filter\Date->valueForm(Array, Object) (Line: 939)
Drupal\views\Plugin\views\filter\FilterPluginBase->buildExposedForm(Array, Object) (Line: 111)
Drupal\views\Form\ViewsExposedForm->buildForm(Array, Object)

Steps to reproduce

This error is triggered when passing an empty value to a query parameter for an exposed views filter, like this:
?date_ranges_start=&date_ranges_start_op=overlaps

Proposed resolution

Improve the isset() check.