The returned value is independant from the fact that CompareContextOnWhichToExecute is null or not.
A code query that relies on a compare context and that is executed without a compare context won't return a useful result.
In such situation, at query runtime an empty NDepend.CodeModel.ICompareContext object is provided, that returns null or false for all properties.

Reference

IQueryCompiledSuccess Interface
IQueryCompiledSuccess Members
ICompareContext Interface
CreateCompareContextWithOlder Method
ExtensionMethodsCQLinqCompare Class
Compile(String,ICompareContext) Method
CompileMany(IEnumerable<String>,ICompareContext) Method
TryCompareSourceWith(ISourceDecl,ISourceDecl) Method
CompareContext Field