I put the e7450 (no GPU) (10.14.6) page login or desktop displayed on the docking station and when the image appears on the monitor I close the cover and can use the docking station (external keyboard, DP or DVI monitor, ...).
No display if boot laptop on the docking station!
Tried with Dell E-Port II Advanced - PR02X (both ports DP or DVI) and Dell E-Port - PR03X