`MethodTypeInferrer.Fix` has potential exposure to `null` compilation

[jcouv]

After annotating MethodTypeInferrer._compilation, it became clear that one of the usage sites is not prepared to deal with null, which could lead to a null dereference.
In practice, I don't think this is reachable at the moment, because a null compilation is used when reducing extension methods and new extension members, and I don't think a function type could contribute to the inference in such a case. But we should confirm and strengthen this code.

            bool fromFunctionType = false;
            if (isFunctionType(best, out var functionType))
            {
                // Realize the type as TDelegate, or Expression<TDelegate> if the type parameter
                // is constrained to System.Linq.Expressions.Expression.
                var resultType = functionType.GetInternalDelegateType();
                if (hasExpressionTypeConstraint(typeParameter))
                {
                    var expressionOfTType = compilation.GetWellKnownType(WellKnownType.System_Linq_Expressions_Expression_T);
                    resultType = expressionOfTType.Construct(resultType);
                }
                best = TypeWithAnnotations.Create(resultType, best.NullableAnnotation);
                fromFunctionType = true;
            }

Found in #80659

[jcouv]

I'm punting this to Backlog because I don't think it's reachable, based on my understanding of the code.
Moreover, if it were reachable, the impact would be limited (only when type Expression<T> is missing) and the crash would likely have been reported already.