Fix for Nested Content to Block List in Grid

[skttl]

When converting grid with DTGE with Nested Content inside, for some reason it runs the migration of Nested Content properties twice. Should probably be fixed in another way, but I got around it, by checking the value, as in this PR.

You could do the check like in Grid to BlockGrid, but I am unsure how it would cope with Nested Content inside of Nested Content.

[KevinJump]

yeah - i need to check the runs twice thing, it's possible we do that in one of the 'prepare' steps but we really shouldn't