Don't throw exceptions in specific HLIL APIs

[fuzyll]

What is the feature you'd like to have?
From an Enterprise user:

Docs say ReferenceSource.hlil may return HighLevelILInstruction or None, which is the behavior I want, but Function.llil (used internally) will raise an ILException if LLIL wasn’t loaded. It would be nice if it returned None without an exception. But, if Exception is the way, can this be reflected in docs?

Is your feature request related to a problem?
Exceptions don’t play nice with map() and filter(), but None does.

Are any alternative solutions acceptable?
I'm not actually sure we should make it not throw an Exception. An alternative way we could solve this is by having e.g. a ReferenceSource.hlil_if_available that, under-the-hood, calls Function.llil_if_available, which (I believe) does what this user wants.

[xusheng6]

I think we should just return None for it. See code at https://github.com/mandiant/capa/blob/2987eeb0acc03e187bdd90bb1644377774a2045a/capa/features/extractors/binja/function.py#L28, where I am aware of .llil may not always give me a result, but the exception throws me off. See mandiant/capa#2249 for more details on this