Fix error in base_list.html.twig  if no "batch" route is available 

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Diff of this commit with white space ignored: https://github.com/pulzarraider/SonataAdminBundle/commit/010c9a92576b58156041483f71859384f3c5e06c?w=1