Set value to null instead of empty string

Author: laurakwhit

src/lib/components/workflow/filter-bar/dropdown-filter-chip.svelte

@@ -326,7 +326,7 @@
         active={localFilter.conditional === option.value}
         on:click={() => {
           if (isNullConditional(option.value)) {
-            localFilter.value = '';
+            localFilter.value = null;
           } else if (isNullFilter) {
             localFilter.value = filter.value;
           }