Fix crash with null callee

[snichols]

Issue

This PR fixes #40942. It's also related to #40943.

Summary

Malformed call expressions that create GDScript::CallNode instances with callee == nullptr are possible. An example of such a malformed expression is var crash = ()(1). When these cases happen during editing, the editor crashes.

The PR fixes the bug by:

1. Adding an accessor GDScript::CallNode::get_callee_type which checks for nullptr and returns Type::NONE in that case.
2. Refactoring the code, where sensible, to use the new accessor.

Additionally, this PR adds a more descriptive error message when empty grouping expressions are encountered.

[vnen]

Your commits are not linked to your GitHub account. This is not a problem for merging but if you want it to appear on your GitHub profile you need to either register the email address you used to commit to your account, or amend the commit authorship to use your GitHub email.

[snichols]

Your commits are not linked to your GitHub account. This is not a problem for merging but if you want it to appear on your GitHub profile you need to either register the email address you used to commit to your account, or amend the commit authorship to use your GitHub email.

Fixed.

[akien-mga]

Thanks! And congrats for your first merged contribution 🎉