Fix for the LoopInvariantConfigurationDialog

[unp1]

Related Issue

This PR fixes a bug that prevented the interactive provision of a loop invariant.

Intended Change

It should be possible to enter loop invariants manually.

Type of pull request

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] Refactoring (behaviour should not change or only minimally change)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] There are changes to the (Java) code
• [ ] There are changes to the taclet rule base
• [ ] There are changes to the deployment/CI infrastructure (gradle, github, ...)
• [ ] Other:

Ensuring quality

• [ ] I made sure that introduced/changed code is well documented (javadoc and inline comments).
• [ ] I made sure that new/changed end-user features are well documented (https://github.com/KeYProject/key-docs).
• [ ] I added new test case(s) for new functionality.
• [x] I have tested the feature as follows: Applied a manually entered loop invariant
• [ ] I have checked that runtime performance has not deteriorated.

Additional information and contact(s)

The problem was if no "free modifies"/"free assignable" was available then creating the whole modifies set "union(freeMod, mod)" led to a NullPointerException. The fix uses "strictly_nothing" as default for free assignables of loops, if non given.

The contributions within this pull request are licensed under GPLv2 (only) for inclusion in KeY.