Add blank to ShippingMethod's TaxCategory select

Addresses https://github.com/spree/spree/issues/4914

Now ShippingMethods whose `tax_category` is `nil` will display a blank selector rather than falsely displaying the first TaxCategory in the system.

Note that due to the UI implementation it appears to be impossible to return to selecting `nil` after a TaxCategory has been selected.
Merge pull request #4956 from brchristian/patch-3

Add blank to ShippingMethod's TaxCategory select