Fix coroutine-related VerifyError

The problem was that we spilled the `origin` variable (see test) as Object, because
we determined the type of merge(null, String) incorrectly.

 #KT-15973 Fixed

compiler/tests-ir-jvm/tests/org/jetbrains/kotlin/codegen/ir/IrBlackBoxCodegenTestGenerated.java

@@ -4799,6 +4799,12 @@ public class IrBlackBoxCodegenTestGenerated extends AbstractIrBlackBoxCodegenTes
             doTest(fileName);
         }
 
+        @TestMetadata("mergeNullAndString.kt")
+        public void testMergeNullAndString() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/box/coroutines/mergeNullAndString.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("multipleInvokeCalls.kt")
         public void testMultipleInvokeCalls() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/box/coroutines/multipleInvokeCalls.kt");