Support SetBenchmark(null) to disable the benchmark

[Martin-Molinero]

Description

QCAlgorithm.SetBenchmark(Symbol) now sets a constant zero benchmark (FuncBenchmark(_ => 0)) when the symbol is null, instead of creating a security benchmark. Since the benchmark is then set, the brokerage model default is not loaded in PostInitialize and no benchmark security is added.

Related Issue

Closes #9854

Motivation and Context

set_benchmark(None) binds to SetBenchmark(Symbol symbol), which passed the null symbol to SecurityBenchmark.CreateInstance and failed with Object reference not set to an instance of an object. This gives users a direct way to disable the benchmark instead of the set_benchmark(lambda x: 0) workaround. From C#, SetBenchmark((Symbol)null) behaves the same.

Requires Documentation Change

No

How Has This Been Tested?

Added AlgorithmBenchmarkTests.NullBenchmarkDisablesTheBenchmark for C# (SetBenchmark((Symbol)null)) and Python (algorithm.set_benchmark(None) called from a Python module). It asserts that after PostInitialize the benchmark is a FuncBenchmark evaluating to 0 and that no securities were added. Both cases fail with the reported null reference without the fix and pass with it.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Refactor (non-breaking change which improves implementation)
• Performance (non-breaking change which improves performance. Please add associated performance test and results)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Non-functional change (xml comments/documentation/etc)

Checklist:

• My code follows the code style of this project.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• My branch follows the naming convention bug-<issue#>-<description> or feature-<issue#>-<description>

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