[FEATURE] Improve test coverage for MLEngineClassLoader class (#3679)

Improved the test coverage by adding the edge case.
Some of the branch coverages which are mssing are unreachable code. The Function, ConnectorExecutor and Ingester values are non null.

Resolves #1377

Signed-off-by: Abdul Muneer Kolarkunnu <[email protected]>

Author: akolarkunnu

ml-algorithms/src/test/java/org/opensearch/ml/engine/MLEngineClassLoaderTests.java

@@ -16,8 +16,10 @@
 import org.opensearch.ml.common.FunctionName;
 import org.opensearch.ml.common.input.Input;
 import org.opensearch.ml.common.input.execute.samplecalculator.LocalSampleCalculatorInput;
+import org.opensearch.ml.common.input.parameter.MLAlgoParams;
 import org.opensearch.ml.common.output.Output;
 import org.opensearch.ml.common.output.execute.samplecalculator.LocalSampleCalculatorOutput;
+import org.opensearch.ml.engine.algorithms.metrics_correlation.MetricsCorrelation;
 import org.opensearch.ml.engine.algorithms.sample.LocalSampleCalculator;
 import org.opensearch.transport.client.Client;
 
@@ -81,4 +83,10 @@ public void testInitInstance_ClassNotFound() {
         MLEngineClassLoader.initInstance("SOMETHING ELSE", null, Object.class);
     }
 
+    @Test
+    public void initInstanceMetricsCorrelationReturnsNull() {
+        MetricsCorrelation instance = MLEngineClassLoader.initInstance(FunctionName.METRICS_CORRELATION, null, MLAlgoParams.class);
+        assertNull(instance);
+    }
+
 }