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/testData/codegen/box/when/enumOptimization/nullIsTheFirstEntry.kt

@@ -0,0 +1,14 @@
+enum class E {
+    A, B;
+}
+
+fun foo(e: E?): String {
+    val c = when (e) {
+        null -> "Fail: null"
+        E.B -> "OK"
+        E.A -> "Fail: A"
+    }
+    return c
+}
+
+fun box(): String = foo(E.B)