Fixes a bug where alias creation would allow `null` for index name, which thereby
applied the alias to _all_ indices. This patch makes the validator throw an
exception if the index is null.
```bash
POST /_aliases
{
"actions": [
{
"add": {
"alias": "empty-alias",
"index": null
}
}
]
}
```
```json
{
"error": "ActionRequestValidationException[Validation Failed: 1: Alias action [add]: [index] may not be null;]",
"status": 400
}
```
The reason this bug wasn't caught by the existing tests is because
the old test for nullness only validated against a cluster which had
zero indices. The null index is translated into "_all", and since
there are no indices, this fails because the index doesn't exist.
So the test passes.
However, as soon as you add an index, "_all" resolves and you get the
situation described in the original bug report: null index is
accepted by the alias, resolves to "_all" and gets applied to everything.
The REST tests, otoh, explicitly tested this bug as a real feature and therefore
passed. The REST tests were modified to change this behavior.
Fixes#7863
f5b2dfd052