Fix bool fields (#778)

* Improved UX for boolean fields

- Use segmented button
- Allow tristate
- Improved filtering options

* Bug fix for null filter values

* Prevent null filters from being sent to the server

* Update release notes

lib/widget/paginator.dart

@@ -92,9 +92,10 @@ abstract class PaginatedSearchState<T extends PaginatedSearchWidget>
 
       // Skip null values
       if (value == null) {
-        continue;
+        f[k] = "null";
+      } else {
+        f[k] = value.toString();
       }
-      f[k] = value.toString();
     }
 
     return f;
@@ -341,7 +342,16 @@ abstract class PaginatedSearchState<T extends PaginatedSearchWidget>
       Map<String, String> f = await constructFilters();
 
       if (f.isNotEmpty) {
-        params.addAll(f);
+        for (String k in f.keys) {
+          // Remove any existing filter keys
+          dynamic value = f[k];
+
+          if (value == null || value == "null") {
+            params.remove(k);
+          } else {
+            params[k] = value.toString();
+          }
+        }
       }
 
       final page = await requestPage(_pageSize, pageKey, params);