Fixed `?search=` empty param after members bulk delete

no issue

- the members query reset was using `""` rather than `null` due to a bug in an old version of search/filtering but that meant we could end up with an empty param
- switched to `null` as it no longer appears necessary to use a blank string, this matches behaviour of other params

app/helpers/reset-query-params.js

@@ -18,7 +18,7 @@ export const DEFAULT_QUERY_PARAMS = {
     'members.index': {
         label: null,
         paid: null,
-        search: '',
+        search: null,
         filter: null,
         order: null
     }

tests/acceptance/members-test.js

@@ -194,8 +194,7 @@ describe('Acceptance: Members', function () {
             expect(find('[data-test-button="confirm"]')).to.not.exist;
 
             // members filter is reset
-            // TODO: fix query params reset for empty strings
-            expect(currentURL()).to.equal('/members?search=');
+            expect(currentURL()).to.equal('/members');
             expect(findAll('[data-test-member]').length).to.equal(6);
 
             await click('[data-test-button="close-modal"]');