Implementer's Guide: XCMP/HRMP watermark in case of no pending messages

[pepyakin]

There are really two options what we do here and in either case the solution would be trivial or at least close to it. Here I am just leaving breadcrumbs for motivation

Options:

1. Allow the watermark to stay the same - basically circumventing the rule of the watermark advancement - if there are no pending messages
2. Always allow the watermark to land on the relay parent block (in addition to the blocks found in the digest)

The first approach requires answering the question: if there are pending messages to a parachain. The second doesn't require any additional checks.

I am inclined towards the second approach