[Transform] NPE when removing tasks without config

[hendrikmuhs]

regression caused by #51072

detected by CI: https://gradle-enterprise.elastic.co/s/af3wrse63ax34/tests/fcijb24gqtmnc-ldayr6b5ykxam

PersistentTasksCustomMetaData is not checked for null

[elasticmachine]

Pinging @elastic/ml-core (:ml/Transform)