LineItems allow a quantity of zero

[RyanofWoods]

Adding a LineItem with a quantity of zero to cart doesn't really make sense. There was discussion on removing the normalization of LineItem's quantity to 0, and enforcing the quantity to be 1 and higher. This could free up several tangential parts of the codebase:

#4177 (comment)

solidus/core/app/models/spree/line_item.rb

Line 25 in d7b8cf9

before_validation :normalize_quantity

solidus/core/app/models/spree/line_item.rb

Lines 137 to 139 in d7b8cf9

	def normalize_quantity
	self.quantity = 0 if quantity.nil? || quantity < 0
	end

[waiting-for-dev]

IIRC, the proposal didn't go through because that could break clients relying on that behavior to remove line items. However, I don't see why we can't go through a regular deprecation cycle. We can even put it on hold until the next major if we see it as critical.

[kennyadsl]

I'm in favor of this change. I also tried to remove it at some point with #4373, but didn't have the proper time to complete the work.

[RyanofWoods]

I am working on the two PRs as we previously discussed @kennyadsl. I should have something in the upcoming weeks.

[jarednorman]

I think everyone agrees with this in principle, but it's a pretty minor issue and would require a deprecation cycle and breaking existing storefronts, so probably not worth pursuing this now that it's stalled.

If someone picks this up and thinks it's worth it, #5275 is the place to start.