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

• 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)
• 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.
• 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]

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 ⚠️

Additional details and impacted files

@@             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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