Crawler default namespace fix

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9732, #6650
| License       | MIT
| Doc PR        | symfony/symfony-docs/pull/2979

Fix backwards compatibility of xml namespaces for having only one default namespace.