Refactor Product#available? to match docs

[brchristian]

Description

This is a very small quality-of-life type improvement, but I noticed that the safe navigation operator can be used to simplify the logic of the Product#available? method.

Rephrasing the logic this way we:

• Have two(ish) conditions instead of three
• Avoid the semantic confusion of "not nil or future"
• Flip the deleted and available_on conditions, to better match the docs, which say,
  A product is available if it has not been deleted and the available_on date is in the past.

It’s not earth shattering by any means, but it makes the code that much more readable IMO, especially for folks new to Solidus.

[spaghetticode]

@brchristian thanks! 👍