Fix generic data class hashCode for null

Null may come from everywhere, so we always check for it (when the property's
type is not primitive)

compiler/testData/codegen/dataClasses/hashcode/genericNull.kt

@@ -0,0 +1,7 @@
+data class A<T>(val t: T)
+
+fun box(): String {
+    val h = A<String?>(null).hashCode()
+    if (h != 0) return "Fail $h"
+    return "OK"
+}