DATABASE UPDATE: fix some columns being NOT NULL while they should be NULL

This MySQL database schema update solves bug 302837. In short, updates
4 -> 5, 5 -> 6, 6 -> 7 and 9 -> 10 ignored NULL status of some columns
and replaced them with NOT NULL columns, causing various consequences,
one of them is Dynamic Collection not working. Fix it back.

A list of columns to fix was obtained by comparing a database created
by Amarok 2.1.1 and then upgraded to current versin with a db freshly
created by Amarok 2.6-git.

Tom, please test that this fixes your problem if you (temporarily)
restore your old database where you were able to reproduce your bug.

BUG: 302837
FIXED-IN: 2.7