[BUG] Different behavior when the setting index.number_of_routing_shards is set to null or not set.

[gaobinlong]

Describe the bug

The index level setting index.number_of_routing_shards can be set at the index creation time, but if it is set to null explicitly, the value is different from the value when it's not set.

Related component

Indexing

To Reproduce

1. Set to null

DELETE abc

PUT abc
{
  "settings": {
    "index.number_of_routing_shards":null,
    "index.number_of_shards":"2"
  }
}

GET _cluster/state/metadata/abc

, we can see routing_num_shards is 2, equals to the number_of_shards, but not correct because we cannot split the index anymore.

2. Not set

DELETE abc

PUT abc
{
  "settings": {
    "index.number_of_shards":"2"
  }
}

GET _cluster/state/metadata/abc

we can see routing_num_shards is 1024, this value is correct.

Expected behavior

When the setting is set to null, the value of routing_num_shards should be same to the value when it's not set.

Additional Details

Plugins
Please list all plugins currently enabled.

Screenshots
If applicable, add screenshots to help explain your problem.

Host/Environment (please complete the following information):

• OS: [MacOS]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.