Allow CompositeBlock::appliesToRight to return null when unsure

Allow CompositeBlock::appliesToRight to return null when unsure

CompositeBlock::appliesToRight checks $block->appliesToRight()
for each of the original blocks from which it is made.

AbstractBlock::appliesToRight returns:
* true if the block applies to the right
* false if the block does not apply to the right
* null if unsure

Before this, CompositeBlock::appliesToRight can only return true
or false. After this, it returns:
* false if false for all of the original blocks
* true if true for one or more original blocks
* null otherwise

Bug: T229417
Bug: T231145
Change-Id: Ie93b7691b57ac6a8f86b3641ad07a1d54babcd42