Only add assumption scope if not empty

[FrNecas]

The GOTO trace may contain instructions from the global scope (e.g. assigning a value to a global variable). In such case, the scope should be global, i.e. no assumption.scope as per the witness specification.

I found this issue during 2LS SV-comp preruns due to witnesslint failing on a singular assumption.scope data tag. There is no scope on global variable definitions, therefore the scope should not be added.

@peterschrammel this should later be cherry-picked to 2LS's fork

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message). [Sorry, I am not very familiar with the code base and don't really know how to test changes around witnesses]
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