* eth/protocols/snap: don't include empty snapshot slot slice

This PR fixes the snapshot storage serving handler. In snap protocol
the response is capped by the response size. Server can cutdown the
response if the accumulated byte size exceeds the local hard limit.

It means we can meet a special scenario that there is no storage slot
included for a requested account, but we attach the proof for this
account by mistake.

So in the prover side, when it meets a empty storage response but with
a valid proof proves there are some more slots left in the trie, then
requestor will reject this response and disconnect with server.

In this PR, if there is no storage slot served for the requested account,
then no proof should be attached as well.

* eth/protocols/snap: loosen restrictions for flaky tests

* eth/catalyst: fix flaky test in catalyst