[Backport 2.17] Fix RestMLInferenceSearchResponseProcessorIT: add skip when aws key is null

[opensearch-trigger-bot]

Backport 93d0429 from #2926

[mingshl]

flaky test

REPRODUCE WITH: gradlew ':opensearch-ml-plugin:integTest' --tests "org.opensearch.ml.rest.RestBedRockInferenceIT.test_bedrock_embedding_model" -Dtests.seed=85CFE7458A9487AF -Dtests.security.manager=false -Dtests.locale=fr-FR -Dtests.timezone=America/Dawson_Creek -Druntime.java=21
org.opensearch.ml.rest.RestBedRockInferenceIT > test_bedrock_embedding_model STANDARD_ERROR
    REPRODUCE WITH: gradlew ':opensearch-ml-plugin:integTest' --tests "org.opensearch.ml.rest.RestBedRockInferenceIT.test_bedrock_embedding_model" -Dtests.seed=85CFE7458A9487AF -Dtests.security.manager=false -Dtests.locale=fr-FR -Dtests.timezone=America/Dawson_Creek -Druntime.java=21

org.opensearch.ml.rest.RestBedRockInferenceIT > test_bedrock_embedding_model FAILED
    java.lang.AssertionError: Failing test case name: without_step_size
        at __randomizedtesting.SeedInfo.seed([85CFE7458A9487AF:A9873A58151B5046]:0)
        at org.junit.Assert.fail(Assert.java:89)
        at org.junit.Assert.assertTrue(Assert.java:42)
        at org.opensearch.ml.rest.RestBedRockInferenceIT.test_bedrock_embedding_model(RestBedRockInferenceIT.java:75)

org.opensearch.ml.rest.RestBedRockInferenceIT > test_bedrock_multimodal_model_empty_imageInput_null_textInput STANDARD_OUT
    [2024-09-10T12:13:02,459][INFO ][o.o.m.r.RestBedRockInferenceIT] [test_bedrock_multimodal_model_empty_imageInput_null_textInput] before test
Error: 24-09-10T12:13:23,483][ERROR][o.o.m.r.MLCommonsRestTestCase] [test_bedrock_multimodal_model_empty_imageInput_null_textInput] method [POST], host [http://127.0.0.1:62596/], URI [/_plugins/_ml/_predict/TEXT_EMBEDDING/mJpa3ZEB91CsNMJB1WwC], status line [HTTP/1.1 400 Bad Request]