Interpreter error message not clear

### Describe the problem you discovered

If we pass a smart contract code with just the following code:

```elixir
@version 1
```

We get an error which is not accurate:

```
unexpected term - @:atom => \"version\"(1) - L1
```

This issue seems to related to a pattern matching issue

https://github.com/archethic-foundation/archethic-node/blob/31cc63def92a24b74a5133f3c8f19a7d3e9a7886/lib/archethic/contracts/interpreter.ex#L36C10-L38C44

### Describe the solution you'd like

The interpreter should accept @version 1 without having a block or mention the contract cannot be empty with only the version mention.

### Epic

#608

Start in lib/archethic/contracts/interpreter.ex at the linked lines 36-38 and reproduce the error with a contract containing only @version 1. Trace the pattern-matching path that produces the inaccurate message, then verify that the interpreter either accepts this declaration without a block or reports clearly that the contract cannot contain only the version declaration.

Written by the indexing model from the issue text.