FFMPEG replace “no video” part with an image

Using FFMPEG, I have merged a video and audio file to WEBM. But the problem is: if there is audio and no video the video file has just some stuck and still images, because there is no video recorded at that timestamp. The video file and audio file have mostly the same duration (In live stream the video track can be switched off and on).

Is there a way to replace these missing video parts with an image, like some jpg etc. or a placeholder with a black screen?

ffmpeg -i video2.webm -i audio2.opus -c:v libvpx -c:a libopus -strict experimental test.webm

Could not find any solutions, all day of searching.