If Base.DEPOT_PATH is empty, Pkg.PlatformEngines.get_server_dir s…

…hould return `nothing` (instead of erroring) (#2808)

(cherry picked from commit 5823469)

src/PlatformEngines.jl

@@ -55,6 +55,7 @@ function get_server_dir(url::AbstractString, server=pkg_server())
         @warn "malformed Pkg server value" server
         return
     end
+    isempty(Base.DEPOT_PATH) && return
     joinpath(depots1(), "servers", String(m.captures[1]))
 end