`get_blueprint_entity_tags` return type should be optional

[The_LORD_thy_GOD]

The following two methods:

https://lua-api.factorio.com/latest/cla ... ntity_tags
https://lua-api.factorio.com/latest/cla ... ntity_tags

are documented as returning non-optional `Tags`. However, they can both return `nil` if the entity in question has no tags, so their return type should be `Tags?` instead.

(I am filing this as a doc issue, but if the function is always supposed to be returning a `Tags` then this may be an actual bug instead.)