Fix LightExpression compound assignment round trips without a conversion

[dajiaohuang]

Problem

A normal integer compound assignment can compile with CompileFast, but converting its LightExpression representation back to System.Linq.Expressions throws NullReferenceException:

using System;
using System.Linq.Expressions;
using FastExpressionCompiler.LightExpression;
using static System.Linq.Expressions.Expression;

var value = Variable(typeof(int), "value");
var source = Lambda<Func<int>>(Block(new[] { value },
    Assign(value, Constant(3)), AddAssign(value, Constant(2)), value));
var light = source.ToLightExpression();
var result = light.ToLambdaExpression().Compile()(); // expected 5; throws before this change

FromSysExpressionConverter creates an OpAssignMethodConversionBinaryExpression through MakeBinary. A compound assignment's conversion lambda is optional; CreateSysExpression unconditionally dereferences it. Preserve an absent conversion when forwarding the arguments to System MakeBinary, as the base binary-expression conversion already does.

Changes

• Preserve the optional conversion rather than dereferencing null.
• Add maintained round-trip tests with and without a conversion lambda, registered in LightExpressionTests.Run().

Validation

• The no-conversion regression throws on master 1d2a4bd364bd5271f5594074f76e298f5afa53db; both regression methods pass with the fix on .NET 8.0.22 and 10.0.0.
• A bounded synthetic consumer fixture passed 108 result checks on both runtimes (including closures, calls, control flow, assignment and System/Light conversion).
• Release build of FastExpressionCompiler.LightExpression.UnitTests for net10.0 passed without warnings/errors.
• BuildScripts/MakeInternal.ps1 completed successfully; generated internal copies are ignored.
• Full upstream runner and net472/netstandard/cross-platform matrix were not run locally. No performance claim.