When you create an expression that is an empty sum, it evaluates to `Val{false}()`

instead of `0`

:

```
model = Model()
@expression(model, sum(1 for i in 1:0))
julia> Val{false}()
```

A similar problem was posted in the GitHub issue Problem with Empty Summations and closed here, but it seems to work only for constraints, not for expressions.

Something even weirder happens when you try to create a collection of such expressions:

```
model = Model()
@expression(model, [i in 1:2], sum(1 for j in 1:0))
julia> ERROR: Collection of expressions with @expression must be linear.
For quadratic expressions, use your own array.
```

Is this a or am I doing something wrong?