JVM IR: fix "null" as the first entry in MappedEnumWhenLowering

If "null" was the first entry in an optimizable "when" over enum,
mapRuntimeEnumEntry was called before mapConstEnumEntry, and the
$WhenMappings field was not created. Now both mapConstEnumEntry and
mapRuntimeEnumEntry create this field on the first access

compiler/tests/org/jetbrains/kotlin/codegen/BlackBoxCodegenTestGenerated.java

@@ -26192,6 +26192,11 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
                 runTest("compiler/testData/codegen/box/when/enumOptimization/nonConstantEnum.kt");
             }
 
+            @TestMetadata("nullIsTheFirstEntry.kt")
+            public void testNullIsTheFirstEntry() throws Exception {
+                runTest("compiler/testData/codegen/box/when/enumOptimization/nullIsTheFirstEntry.kt");
+            }
+
             @TestMetadata("nullability.kt")
             public void testNullability() throws Exception {
                 runTest("compiler/testData/codegen/box/when/enumOptimization/nullability.kt");