plasma5: Fix setup hook some more
plasma5: Fix setup hook some more

1. `[[...]]` is more robust wrt empty expansions than `[..]`

2. `${foo-}` is simpler and won't accidentally be a url compared to
`${foo:-}`

3. `+=` works even when the variable is undefined.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs