FileSystemStorage no longer accepts reverse_lazy as base_url

[tpazderka]

Changes introduced with commit fb9d8f06 fixing #22717 causes FileSystemStorage to evaluate the content of base_url uppon initialization.
This causes issues if the base_url is defined as reverse_lazy as it is resolved and can cause ImportError.

I think that the laziness of the base_url should be preserved. Either the check for the trailing slash can me moved to path generation or the __init__ can be updated to expect reverse_lazy as base_url.