Unnamed return variable is displayed incorrectly in counter example

## Description

When the return value is unnamed, the left side of the equality sign in the counterexample appears empty.

```solidity
contract C {
function f(uint256 value) public payable returns(bool) {
value = value / value;
}
}
```

Compiled with `solc --model-checker-engine all a.sol`:

```
Warning: CHC: Division by zero happens here.
Counterexample:

value = 0
= false // The left side of the equality sign here is empty.

Transaction trace:
C.constructor()
C.f(0){ msg.value: 19 }
--> out.sol:3:13:
|
3 | value = value / value;
| ^^^^^^^^^^^^^
```

## Environment

- Compiler version: 0.8.26

Start by reproducing the issue with `solc --model-checker-engine all a.sol` using the Solidity example in the report, then trace the model-checker counterexample output. Done means the unnamed return value no longer produces an empty left-hand side in the counterexample, with regression coverage for this case.

Written by the indexing model from the issue text.