The calculator should always return a BigDecimal, but that's not always the case.

For example the flat_rate calculator return 0 as Integer on the else branch.

If the calculator returns a type other than BigDecimal a warning will be issued by the adjustment action for the promotions,
for example create_quantity_adjustments.

I think that we should document this expected interface on the Calculator, and update our actual implementation to respect the expected interface.

I wasn't able to find a test that raises this warning on the Solidus codebase, we should probably add some tests to cover this case.

Solidus Version:
First found out on Solidus 2.5.2.

To Reproduce

• Write a test that creates a create_quantity_adjustments using a flat_rate calculator giving in input a nil object.
• Launch it.
• A Warning should appear in the console about results not being a BigDecimal. ("Spree::Calculator::FlatRate#compute returned 0, it should return a BigDecimal")

Current behavior

• Raises a warning when using a standard calculator.

Expected behavior

• Doesn't raise any warning when using a standard calculator.