When upgrading data nodes to a newer version before
master nodes there was a risk that a transform running
on an upgraded data node would index a document into
the new transforms internal index before its index
template was created. This would cause the index to
be created with entirely dynamic mappings.
This change introduces a check before indexing any
internal transforms document to ensure that the required
index template exists and create it if it doesn't.
Backport of #46553
07a0140260