fix(roles): NaN rate values in the RoleForm

* Fix a bug with the `IntegerField` controls of the `RoleForm`,
  that was occurring when user trying to increase/decrease
  a number field which initially has `null`.
* This was causing `NaN` values in the field.
* This fix starts `IntegerField` controls with `0`,
  in case the user is trying to increase/decrease a `null` field.

Addresses https://github.com/topcoder-platform/taas-app/issues/421

Author: cagdas001

src/routes/Roles/components/RoleForm/index.jsx

@@ -89,6 +89,14 @@ function RoleForm() {
 
   const editRate = useCallback(
     (index, changes) => {
+      // a num field is `null` but user trying to increase/decrease it
+      // start with 0
+      for (const key in changes) {
+        if (isNaN(changes[key])) {
+          changes[key] = 0;
+        }
+      }
+      // apply changes
       dispatch(
         setModalRole({
           ...roleState,