Fix multiple NullReferenceException issues - doubles success rate to 62%

[piplarsson]

Summary

This PR fixes critical NullReferenceException bugs that caused Unluau to crash on many real-world Luau bytecode files. The success rate is improved from ~30% to ~62%.

Problem

• Unluau crashed with NullReferenceException when encountering unsupported bytecode instructions
• Only small files (<3KB) could be decompiled successfully
• Success rate was only 30% on real-world files

Solution

Added null checks in 9 critical locations where the decompiler attempted to use null values:

Files Modified:

• src/Unluau/Lifter/Lifter.cs - Added null checks in SETTABLEKS, SETTABLE, NAMECALL handlers
• src/Unluau/Lifter/Statements/IfElse.cs - Check Condition and IfBody for null
• src/Unluau/Lifter/Expressions/NameIndex.cs - Check Expression for null
• src/Unluau/Lifter/Statements/LocalAssignment.cs - Check Expression and Value for null
• src/Unluau/Lifter/Statements/Assignment.cs - Check Variable and Value for null
• src/Unluau/Lifter/Expressions/BinaryExpression.cs - Check Left and Right for null
• src/Unluau/Lifter/Statements/Return.cs - Handle null expressions in list
• src/Unluau/Lifter/Statements/Statement.cs - Check for null statements in sequence
• src/Unluau/Lifter/Expressions/Closure.cs - Check Block for null

Results

Before:

• ✅ 4/13 files (30% success rate)
• ❌ Frequent NullReferenceException crashes
• ❌ Max file size: ~3KB

After:

• ✅ 8/13 files (62% success rate)
• ✅ No crashes - graceful handling of unsupported instructions
• ✅ Max file size: 19.74KB
• ✅ Files with JUMP, FORNLOOP instructions now produce partial output instead of crashing

Testing

Tested with 13 real-world Luau bytecode files ranging from 1.5KB to 53KB.
Test files are available in issue #[71].

Notes

The 5 remaining files that don't decompile appear to use bytecode instructions that Unluau doesn't support yet (JUMP, FORNLOOP, etc.), but at least they no longer crash the decompiler.

Fixes #[72]