Cache null values in XML documentation cache

[N-Olbert]

Summary of the changes (Less than 80 chars)

• Support caching null values when XML documentation files are not found.
  This ensures that failed lookups are also cached, preventing repeated execution of the relatively expensive lookup logic.
• I discovered this while experimenting with the source generator’s XML documentation inference: The frequent lookups result in measurable overhead during startup, because for some actions the XmlDocumentationProvider is still invoked and attempts to load the documentation file. If there is no documentation file at all (for whatever reason, f. e. within a test assembly), the lookup result is not cached, resulting in repeated lookup attempts for every symbol of the assembly.

Closes #bugnumber (in this specific format)