dev/financial#162 Simplify decision as to whether to use a pdf on membership emails

dev/financial#162 Simplify decision as to whether to use a pdf on membership emails

Currently the code that determines whether to attach a pdf for membership emails only attaches it as a pdf if the tax amount is non-zero
This seems both confusing and pointless  - the proposal here is to send as a pdf as long as

is_email_pdf is true and
invoicing is enabled

I feel like 1 and 2 could be separated but they are currently linked so I would suggest any proposals to change that are a follow up