Prevent Dependency PO generation for void methods

[Drodt]

Related Issue

This pull request fixes #3411.

Intended Change

KeY will report an exception when the user attempts to open a proof obligation for void methods.

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

• [x] 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.
• [ ] I have tested the feature as follows: ...
• [ ] I have checked that runtime performance has not deteriorated.

Additional information and contact(s)

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

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (1fb0c10) 37.77% compared to head (678e1da) 37.77%.

Files	Patch %	Lines
.../uka/ilkd/key/proof/init/DependencyContractPO.java	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3422 +/- ## ============================================ - Coverage 37.77% 37.77% -0.01% Complexity 17031 17031 ============================================ Files 2076 2076 Lines 126950 126952 +2 Branches 21381 21382 +1 ============================================ Hits 47952 47952 - Misses 73092 73094 +2 Partials 5906 5906 ```

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