Bug 42981: Prevent toLowerCase to fail when comparing int and string

If stateSave is turned on and one of the option has a value with _id set
to an Integer, toLowerCase is not available and a JS error is raised:
 Uncaught (in promise) TypeError: this._id.toLowerCase is not a function

Test plan:
1.) Admin - table settings: cgi-bin/koha/admin/columns_settings.pl#eholdings|packages
Activate checkbox "Save search state on page change"

2.) Go to ERM : cgi-bin/koha/erm/eholdings/local/packages
And create two packages:
 - One package with My Vendor assigned.
 - One package without a vendor.

3.) Filter the package results list by My Vendor. --> List is filtered to one package

4.) Leave the page, for example by navigating to Agreements.
Return to the Packages list and you will see the issue  (See also attached Screenshot)

=> Without the patch
The package results list becomes unresponsive / hangs after returning to the page.

=> With the patch
The package results list should load and display the filtered results correctly.

Signed-off-by: Michaela Sieber <michaela.sieber@kit.edu>
Signed-off-by: Matt Blenkinsop <matt.blenkinsop@openfifth.co.uk>
Signed-off-by: Pedro Amorim <pedro.amorim@openfifth.co.uk>

koha-tmpl/intranet-tmpl/prog/js/datatables.js

@@ -1163,7 +1163,10 @@ function _dt_add_filters(table_node, table_dt, filters_options = {}) {
                         if (
                             existing_search === this._id ||
                             (existing_search &&
-                                this._id.toLowerCase().match(existing_search))
+                                this._id
+                                    .toString()
+                                    .toLowerCase()
+                                    .match(existing_search))
                         ) {
                             o.prop("selected", "selected");
                         }