Fix NPE in data class 'equals' codegen

Do not perform the optimization from 0a11385006 for data class
properties of nullable types

compiler/testData/codegen/box/dataClasses/equals/null.kt

@@ -0,0 +1,15 @@
+data class A(val v: Any?)
+
+data class B<T>(val v: T)
+
+fun box(): String {
+    val a1 = A(null)
+    val a2 = A("")
+    if (a1 == a2 || a2 == a1) return "Fail 1"
+
+    val b1 = B(null)
+    val b2 = B("")
+    if (b1 == b2 || b2 == b1) return "Fail 2"
+
+    return "OK"
+}