WDB partbyenum misses null symbols #690
Description
When using WDB in partbyenum mode, we are using the integer enumeration values of symbols for the partition names. By default, null symbols will be enumerated to 0N and therefore we end up saving these outside of any integer partition, and then they don't get saved to the HDB at the end of the day.
Bearing in mind that the IDB needs to be able to read the int partitioned db written by WDB, I don't think there is a way in kdb to have an int partition with value of 0N.
I think the way we can fix this is to replace null symbols with some placeholder e.g. `NONE before enumerating/writing, then setting it back to null during EOD merge.
We'd also need to handle this in the maptoint function in the IDB, so that users are still able to query the null symbol.