Don't set Spree.user_class unless it's blank

[dylanfisher]

This line was changed March 28, 2022 and overrides the Spree.user_class, even if it's already been set to a custom user in the host app's Spree.rb file https://github.com/spree/spree_auth_devise/commit/0a45ca07cdb0976281500ccfd231483d3a6c0611#diff-8d002d054349f039bafc03785a785a1594ef6b4539298f59e84e95217a97abedL17.

I don't see why this would be desirable, so I suggest reverting it.