Explorer node: It should not return CAs for blocks that don't exist yet

[dbadol]

Current behaviour
When requesting 'assets' for a block height that does not exist yet, the node explorer returns the list of CAs in the current block height. That's not correct, as by the time we will actually reach that block height, new assets might have been created.

Also, that behavior is not consistent with the 'block' request that returns nothing for block heights that don't exist yet.

Expected behaviour
I believe it would be more correct to return nothing for block heights that don't exist yet.