Handle unsupported media type properly for streaming requests

[reta]

Description

Handle unsupported media type properly for streaming requests

Related Issues

Fixes the NPE in RestBulkStreamingAction when content type header (media type) is not specified:

java.lang.NullPointerException: Cannot invoke "org.opensearch.core.xcontent.MediaType.mediaTypeWithoutParameters()" because "mediaType" is null                    
        at org.opensearch.rest.action.document.RestBulkStreamingAction.lambda$prepareRequest$6(RestBulkStreamingAction.java:127) ~[opensearch-3.0.0-SNAPSHOT.jar:3.0.0-SNAPSHOT]                                                                                                                                                                    
        at org.opensearch.rest.action.document.RestBulkStreamingAction.lambda$prepareRequest$7(RestBulkStreamingAction.java:213) ~[opensearch-3.0.0-SNAPSHOT.jar:3.0.0-SNAPSHOT]                                                                                                                                                                    
        at org.opensearch.rest.BaseRestHandler.handleRequest(BaseRestHandler.java:128) ~[opensearch-3.0.0-SNAPSHOT.jar:3.0.0-SNAPSHOT]                                    
        at org.opensearch.rest.RestController.dispatchRequest(RestController.java:383) [opensearch-3.0.0-SNAPSHOT.jar:3.0.0-SNAPSHOT]                              
        at org.opensearch.rest.RestController.tryAllHandlers(RestController.java:474) [opensearch-3.0.0-SNAPSHOT.jar:3.0.0-SNAPSHOT]        

How to reproduce:

$ curl -X POST http://localhost:9200/_bulk/stream  -d '
{ "delete": { "_index": "movies", "_id": "tt2229499" } }
'

now returns:

{"error":"Content-Type header [application/x-www-form-urlencoded] is not supported","status":406}

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

❌ Gradle check result for d81b1d4: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❌ Gradle check result for d81b1d4: null

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❕ Gradle check result for d81b1d4: UNSTABLE

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 71.90%. Comparing base (1d5082e) to head (d81b1d4).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
.../main/java/org/opensearch/rest/RestController.java	25.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #15531      +/-   ##
============================================
- Coverage     72.01%   71.90%   -0.12%     
- Complexity    63603    63605       +2     
============================================
  Files          5247     5247              
  Lines        297186   297189       +3     
  Branches      42939    42940       +1     
============================================
- Hits         214023   213683     -340     
- Misses        65610    66023     +413     
+ Partials      17553    17483      -70     

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[reta]

@andrross @dblock minor fix for possible NPE, would appreciate a look, thank you!