Remove redundant XCMs from dry run's forwarded xcms (#5913)

# Description

This PR addresses
https://github.com/paritytech/polkadot-sdk/issues/5878.

After dry running an xcm on asset hub, we had redundant xcms showing up
in the `forwarded_xcms` field of the dry run effects returned.
These were caused by two things:
- The `UpwardMessageSender` router always added an element even if there
were no messages.
- The two routers on asset hub westend related to bridging (to rococo
and sepolia) getting the message from their queues when their queues is
actually the same xcmp queue that was already contemplated.

In order to fix this, we check for no messages in UMP and clear the
implementation of `InspectMessageQueues` for these bridging routers.
Keep in mind that the bridged message is still sent, as normal via the
xcmp-queue to Bridge Hub.
To keep on dry-running the journey of the message, the next hop to
dry-run is Bridge Hub.
That'll be tackled in a different PR.

Added a test in `bridge-hub-westend-integration-tests` and
`bridge-hub-rococo-integration-tests` that show that dry-running a
transfer across the bridge from asset hub results in one and only one
message sent to bridge hub.

## TODO
- [x] Functionality
- [x] Test

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Co-authored-by: command-bot <>