Update the minimum number of inputs returned from selectCoins.

Update the minimum number of inputs from `0` to `1`.

The corresponding `ApiCoinSelection` type specifies a `NonEmpty` list of
`ApiCoinSelectionInput` values:

```
data ApiCoinSelection (n :: NetworkDiscriminant) = ApiCoinSelection
    { inputs :: !(NonEmpty (ApiCoinSelectionInput n))
    , outputs :: ![ApiCoinSelectionOutput n]
    , change :: ![ApiCoinSelectionChange n]
    , certificates :: Maybe (NonEmpty ApiCertificate)
    } deriving (Eq, Generic, Show)
```

specifications/api/swagger.yaml

@@ -641,7 +641,7 @@ x-transactionChange: &transactionChange
 x-transactionResolvedInputs: &transactionResolvedInputs
   description: A list of transaction inputs
   type: array
-  minItems: 0
+  minItems: 1
   items:
     type: object
     required: