add missing null checks to public constructors

[scottcarr]

Hello, I'm working on a static analysis tool that looks for missing null checks. It suggested you add these checks:

Reasons:

ActivationCache constructor: if cachePruner is null, the constructor will always throw an exception

Pipeline constructor: fs activationCache is null and subsequently Activate or Deactivate is called, it will throw an exception (because Activate and Deactive both dereference activationCache)

ActivationCacheStrategy constructor: if activationCache is null and subsequently Activate or Deactivate is called, it will throw an exception (because Activate and Deactive both dereference activationCache)

CompiledModuleLoaderPlugin constructor: if assemblyNameRetriever is null and subsequently LoadModules is called, it will throw an exception because LoadModules dereferences assemblyNameRetriever