Modpack installer fails to create database due to invalid SQL

[daavko]

Describe the bug
In 471b6c3, all instances of NULL were changed to nullptr. Unfortunately this seems to have been done with a simple search&replace, which also accidentally changed occurences of NULL within SQL embedded in modpack installer code, causing the SQL to be invalid. As a result, the modpack installer fails to create its sqlite table, since NOT nullptr isn't valid SQL syntax.

Those two lines are affected: https://github.com/longturn/freeciv21/blob/master/tools/fcmp/mpdb.cpp#L167-L168

The fix is to turn nullptr back to NULL for those two lines.

Additional context
Discord conversation https://discord.com/channels/378908274113904641/912500712833974322/1192045288723058758