[BUG] Use of only one scope, instead of "global" and another, is impossible.

[tvoigtlaender]

Describe the bug
Only using the "global" scope or exclusively using non-"global" scopes leads to errors.
A "Variable not defined"-error occurs if only the "global" scope and no non-"global" scopes are declared.
The code generation will attempt to include a "global" scope, even if there are no producers for it.

To Reproduce
Reduce any existing config to either only include the "global" or any non-"global" scope.

Expected behavior
It should be possible to run CROWN with only a single scope, either only global or only a non-global scope.

Additional context
The case for not having a global scope is already included in the code generation logic, but is currently hard coded to not be used (

CROWN/code_generation/configuration.py

Line 92 in 1f1baba

self.global_scope = "global"

)