When creating a block through the API it is possible to use any string as part of the page restrictions. Using a non-existing Title results in an entry on ipblocks_restrictions with ir_value = 0
In my opinion SpecialBlock::processForm() should fail o return with a warning if non-existing titles are being used as part of the page restrictions and PageRestriction should not be saved if ir_value is 0
Note: this issue does not fail on save. It fails while editing the block through SpecialBlock