Fix NRE in CollectDeclaredReferencesTask when ResolvedReferences is null

[dfederm]

Fixes an ArgumentNullException from CollectDeclaredReferencesTask.Execute() observed on Substrate vcxproj builds with ReferenceTrimmer 3.3.12+:

error MSB4018: The "CollectDeclaredReferencesTask" task failed unexpectedly.
System.ArgumentNullException: Value cannot be null. Parameter name: source
   at System.Linq.Enumerable.SingleOrDefault[TSource](IEnumerable`1 source, Func`2 predicate)
   at ReferenceTrimmer.Tasks.CollectDeclaredReferencesTask.Execute()

Root cause

ResolvedReferences is declared ITaskItem[]? but was dereferenced unconditionally on the referenceHintPath / referenceSpec fallback path. On vcxproj projects, _ReferenceTrimmerResolvedReferences (sourced from ReferencePathWithRefAssemblies) is empty/uninitialized and MSBuild passes null to the task instead of an empty array → NRE.

Changes

• CollectDeclaredReferencesTask.cs:
  • Guard ResolvedReferences?.SingleOrDefault(...) with the null-conditional operator.
  • Audit fix in GetPackageInfos(): short-circuit when ProjectAssetsFile is null/missing and when LockFileUtilities.GetLockFile returns null (legacy or non-SDK projects can have PackageReferences without a project.assets.json).
• New unit tests (CollectDeclaredReferencesTaskTests.cs) drive the task directly with null inputs:
  • All collection inputs null.
  • The vcxproj repro: a Reference whose path can't be resolved locally with ResolvedReferences = null.
  • PackageReferences non-null with ProjectAssetsFile = null.
• Tests project now references Tasks so the task can be unit-tested; small MockBuildEngine / MockTaskItem helpers added.

All other nullable collection inputs in Execute() (References, ProjectReferences, PackageReferences, IgnorePackageBuildFiles, TargetFrameworkDirectories) are already null-checked.