[ML] Transform source index view is empty if remote indices are on a remote CCS 6.8 cluster

[wwang500]

Build: 7.12 GA build

Step to reproduce:

1. Setup a CCS connection to a remote 6.8 cluster.
2. On the remote 6.8 cluster, load Kibana sample data ecommerce index
3. On 7.12 cluster, create an index pattern called 6_8:kibana_sample_data_ecommerce
4. From transform ui, create transform using that index pattern

Observed:
source index view is empty, also, if sort on a field, the whole UI turns very slow and not responsive.

Expected:
source index view is not empty, and should work as normal.

[elasticmachine]

Pinging @elastic/ml-ui (:ml)