`Serial.print` broken for `uint64 == 0`

Repro:

```
uint64_t zero = 0;
uint64_t one = 1;

Serial.println("----");
Serial.println(zero);
Serial.println(one);
Serial.println("----");
```

outputs

```
----

1
----
```

Bug lies in `Print.cpp` inside `printULLNumber()`