Fix issues found during testing of the `Iterator` implementation for `StorageVec`

[ironcev]

The tests written in #6821 fails for types (), [u64;0], and the EmptyStruct.

Using () causes an ICE:

error
   --> /.../sway/sway-lib-std/src/option.sw:191:15
    |
189 |     /// ```
190 |     pub fn unwrap(self) -> T {
191 |         match self {
    |               ^^^^ Internal compiler error: Failed to get variant type from enum in `unsigned downcast`.
Please file an issue on the repository and include the code that triggered this error.
192 |             Self::Some(inner_value) => inner_value,
193 |             _ => revert(0),
    |

The other two types are zero-sized, and we've already had various issues with such types in the past.

Because of that and the fact that all the tests are passing for all other tested types, it is reasonable to assume that the issues have nothing to do with the implementation of the Iterator trait, but are rather some underlying already existing issues.