defaultIfEmpty() possibly buggy...

Hello there!

I'm getting
`java.lang.RuntimeException: onNext called with null. Null values are generally not allowed in 2.x operators and sources.`
When the select doesn't have any retuned value (`null`), shouldn't it default to -1 instead of passing on `null`?
```
return database
.select("SELECT SUM(amount) FROM table")
.parameter("param1", param1)
.parameter("param2", param2)
.getAs(BigDecimal.class)
.defaultIfEmpty(new BigDecimal(-1));
```

Once again thank you thank you for your time!