Do not generate CHECKCAST after 'null' literal

#KT-15411 Fixed

compiler/testData/codegen/bytecodeText/checkcast/kt15411.kt

@@ -0,0 +1,15 @@
+// KT-15411 Unnecessary CHECKCAST bytecode when dealing with null
+
+fun test1(): String? {
+    return null
+}
+
+fun test2(): BooleanArray? {
+    return null
+}
+
+fun test3(): Unit? {
+    return null
+}
+
+// 0 CHECKCAST