Wrong breadcrumb link when the filter key is empty

Yeah, we're going to be testing against 8.4.x only from now on. I already fixed that fail in #2877989: Module depends on a search api service and it shouldn't, committing that soon.

The other changes look very good, thanks for the test. That one also looks good. I do have some nits to pick, sorry about that.

1. +++ b/tests/src/Functional/BreadcrumbIntegrationTest.php
   @@ -48,16 +48,7 @@ public function setUp() {
        $name = 'Keywords';
   
   @@ -71,6 +62,42 @@ public function testGroupingIntegration() {
   +    $this->editFacetConfig(['filter_key' => $this->randomMachineName()]);
   

   We try not to use the random* functions in our testsuite. It could lead to testfails that are really hard to reproduce, so let's not do that and use a fixed value here instead.

2. +++ b/tests/src/Functional/BreadcrumbIntegrationTest.php
   @@ -71,6 +62,42 @@ public function testGroupingIntegration() {
   +   * @param array $config
   +   */
   

   Let's document this clearer, something like: "The new configuration for the facet"

3. +++ b/tests/src/Functional/BreadcrumbIntegrationTest.php
   @@ -71,6 +62,42 @@ public function testGroupingIntegration() {
   +  public function breadcrumbTest() {
   +
        // Breadcrumb should show Keywords: orange > Type: article, item
   

   Let's remove the blank line in the test here, and also - this doesn't need to be a public method. Let's make it protected.

Committed and pushed, thanks for the patch and the test, wonderful!