Use Logic Op to process missing media in clips set to No Media, based on the input tab it is connected to.
To access the Logic Op menu, use:
Batch, then select a node from the Node bin.
Timeline, then use Batch FX (See Creating Batch FX).
Tools, then select from the menu.
The Modular Keyer, then select a node from the Node bin.
This node accepts a front, back, and matte clip, and outpus a result and an outmatte.