Fix support for models with custom objective functions (Issue #92)

[siboehm]

Fixes #92

Summary

• Adds support for models with custom objective functions where the objective= line is missing from the model file
• Makes the 'objective' key nullable in the scanner
• Properly handles missing objectives in the parser

Changes Made

1. Updated lleaves/compiler/ast/scanner.py to make the 'objective' key nullable by adding null_ok=True
2. Modified lleaves/compiler/ast/parser.py to handle cases where the objective is not present in the scanned model
3. Added test case test_custom_objective to verify custom objective support

Test plan

• Added test for custom objective model
• Run existing test suite to ensure no regressions
• Verify fix with user's actual custom objective model

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