Fix error handling for unknown sorts

[MarcoScaletta]

Intended Change

This change handle properly the error of unknown sort, for example seq in seq::seqGet(seqEmpty, 0).
Previously this would throw a NullPointerException (Message: Given sort is null) after creating the function name null::seqGet and after trying to create AbstractSortedOperator with null sort.

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: ParserExceptionTest, cases: unknownsort.key,unknownsort2.key
• 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.