Remove unnecessary panic for unreachable code

[dejankos]

This PR removes unnecessary panic for unreachable code
and unifies behavior for lazy and eager data fetching.

Although std::unreachable!() could be used here and
serve as self explanatory there is no reason for keeping
this behavior in a lib API that returns a Result type.

Test is rewritten since as far as I can see it was just a c/p where
no data was fetched in loop but code would panic on first
fetchone fn invocations since result was explicitly set to None.

[gitbuda]

@dejankos Nice! I'll check the code a bit later but overall seems reasonable. Thank you!! 🥇

[gitbuda]

cargo fmt changed the two suggestions, so please apply them if that makes sense. As soon as that gets fixed, the PR is ready to be merged.

There is a failing test:

But I've noticed that a couple of days ago, I think it's nothing related to this change. There has to be another PR fixing that.

[dejankos]

@gitbuda Ah yes sorry about fmt, changes accepted.
For that test since attributes size is asserted instead of attribute names I can only guess it's related to running Memgraph version.

[gitbuda]

Great! Thank you very much! I'll merge soon.