fix cannot specify model access control parameters error

[ylwu-amzn]

Description

See such error when register pretrained model without model group id,

{
	"error": {
		"root_cause": [
			{
				"type": "illegal_argument_exception",
				"reason": "You cannot specify model access control parameters because the Security plugin or model access control is disabled on your cluster."
			}
		],
		"type": "illegal_argument_exception",
		"reason": "You cannot specify model access control parameters because the Security plugin or model access control is disabled on your cluster."
	},
	"status": 400
}

The reason is input.getBackendRoles() is not null, but it's empty.
This PR fixed the bug.

Issues Resolved

[List any issues this PR will resolve]

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff

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[zane-neo]

Seems strange, we better not allow backend_roles field show up in user's request instead of passing a empty list, can you change to this behavior?

[ylwu-amzn]

Thanks, think the same. I submit a new commit which add unit test for model group and fix some bug

[zane-neo]

I think we can change this back to input.getBackEndRoles() != null, right?

[ylwu-amzn]

I think keep as CollectionUtils.isEmpty is safer. Do you see any bug if we keep CollectionUtils.isEmpty?

[zane-neo]

I mean we better make the field not showing up when access control is not enabled, user can pass the backend_roles field with empty list when using CollectionUtils.isEmpty, I prefer to make the field not shown.

[codecov]

Codecov Report

Merging #1068 (9012a96) into 2.x (1602aea) will increase coverage by 0.01%.
The diff coverage is 33.33%.

❗ Current head 9012a96 differs from pull request most recent head e20dc0b. Consider uploading reports for the commit e20dc0b to get more accurate results

@@             Coverage Diff              @@
##                2.x    #1068      +/-   ##
============================================
+ Coverage     77.32%   77.33%   +0.01%     
- Complexity     2052     2054       +2     
============================================
  Files           167      167              
  Lines          8532     8534       +2     
  Branches        846      848       +2     
============================================
+ Hits           6597     6600       +3     
+ Misses         1557     1555       -2     
- Partials        378      379       +1     

Flag	Coverage Δ
ml-commons	77.33% <33.33%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...a/org/opensearch/ml/model/MLModelGroupManager.java	45.53% <33.33%> (+0.08%)	⬆️

... and 2 files with indirect coverage changes