Add NullPointerException for Java synchronized blocks with null monitor

[tautschnig]

Implement null checking for monitorenter/monitorexit bytecode instructions to throw NullPointerException when the monitor object is null, as per Java bytecode specification.

Fixes: #1236

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• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a 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).
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• My PR is restricted to a single feature or bugfix.
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[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.00%. Comparing base (1505c36) to head (f5f00d1).

Files with missing lines	Patch %	Lines
...src/java_bytecode/java_bytecode_convert_method.cpp	90.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8757   +/-   ##
========================================
  Coverage    80.00%   80.00%           
========================================
  Files         1700     1700           
  Lines       188254   188272   +18     
  Branches        73       73           
========================================
+ Hits        150615   150631   +16     
- Misses       37639    37641    +2     

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