Show LBTC / L-sat if a Liquid wallet has only LBTC

### Background

The current logic on master is to show BTC/sat if there are no Liquid assets, and show a select box with all assets and LBTC if ther are any assets:


If a Liquid wallet has only LBTC the UI displays this:

It would be better to changes the units, like so:


### Approach

In `recipient-box.html ` add an `elif` (`{% elif specter.is_liquid and wallet.balance["assets"] == {} %}`) in `addAssetSelectorToShadowRoot()` and adjust `updateUnitLabelAndStep()`

Start in recipient-box.html and inspect addAssetSelectorToShadowRoot() and updateUnitLabelAndStep(). Reproduce the case of a Liquid wallet whose assets mapping is empty but whose balance is LBTC, then verify the UI shows LBTC and L-sat units instead of the asset selector.

Written by the indexing model from the issue text.