fix autoconversion bug

attention with autoconversions around `Option[Int]` and similar:

```
scala> Option(null: Integer)
val res0: Option[Integer] = None

scala> val o: Option[Int] = Option(null: Integer)
val o: Option[Int] = Some(0)

scala> val o: Option[Int] = Option(null: Integer).asInstanceOf[Option[Int]]
val o: Option[Int] = None
```

Author: mpollmeier

codegen/src/main/scala/overflowdb/codegen/CodeGen.scala

@@ -1826,7 +1826,7 @@ class CodeGen(schema: Schema) {
 
             case Cardinality.ZeroOrOne =>
               s"""def $name(value: $valueTypeNullable): this.type = {
-                 |  this.$name = Option(value)
+                 |  this.$name = Option(value).asInstanceOf[Option[$valueType]]
                  |  this
                  |}
                  |