Fix codegen of as-casts for nullable generics

compiler/testData/codegen/box/casts/castGenericNull.kt

@@ -0,0 +1,13 @@
+fun castToString<T>(t: T) {
+    t as String
+}
+
+
+fun box(): String {
+    try {
+        castToString<Any?>(null)
+    } catch (e: Exception) {
+        return "OK"
+    }
+    return "Fail"
+}