[BUG] "." as field name yields array_index_out_of_bounds_exception

[shdubsh]

Describe the bug

"." as field name yields array_index_out_of_bounds_exception

Related component

Indexing

To Reproduce

Dev Tools:

POST index_name/_doc
{".":"foo"}

Yields

{
  "error": {
    "root_cause": [
      {
        "type": "mapper_parsing_exception",
        "reason": "failed to parse"
      }
    ],
    "type": "mapper_parsing_exception",
    "reason": "failed to parse",
    "caused_by": {
      "type": "array_index_out_of_bounds_exception",
      "reason": "Index -1 out of bounds for length 0"
    }
  },
  "status": 400
}

Expected behavior

We could either index the document or yield a better error message.

Additional Details

Plugins
Defaults in docker

Screenshots
n/a

Host/Environment (please complete the following information):

• OS: n/a - using docker.io/opensearchproject/opensearch:latest
• Version - affects 2.15.0 and earlier

Additional context
n/a

[gaobinlong]

The exception is thrown here because subfields is empty in this case:

OpenSearch/server/src/main/java/org/opensearch/index/mapper/DocumentParser.java

Line 1045 in 0cde7ba

return objectMapper.getMapper(subfields[subfields.length - 1]);

, since . is reserved for object resolution, we can fail the indexing request when . is used as a field name, @shdubsh mind opening a PR for this bug?