fix: handle empty JULIAHUB_PROJECT_UUID var correctly

Author: pfitzseb

src/authentication.jl

@@ -515,6 +515,8 @@ function _juliahub_project(
     elseif isa(project, UUIDs.UUID)
         return project
     elseif isa(project, AbstractString)
+        isempty(project) && return nothing
+        
         project_uuid = tryparse(UUIDs.UUID, project)
         if isnothing(project_uuid)
             throw(