do not pass NULL as CStrings to callbackSwift as the following CString-to-Swift conversion crashes on getting a nil-pointer

deltachat-ios/DC/wrapper.c

@@ -4,6 +4,6 @@ void callbackSwift(int, long, long, const char*, const char*);
 
 uintptr_t callback_ios(dc_context_t* mailbox, int event, uintptr_t data1, uintptr_t data2)
 {
-    callbackSwift(event, data1, data2, (const char*)data1, (const char*)data2);
+    callbackSwift(event, data1, data2, data1? (const char*)data1 : "", data2? (const char*)data2 : "");
     return 0;
 }