Advice on re-grouting this shower

The caulking in my shower was starting to grow some mould so I have removed it and discovered the grout in the corners of the shower where two walls meet or where floor meets wall are in bad shape or entirely missing.

I am also a bit concerned by the fact that there doesn't appear to be anything immediately behind this grout in the corner. This was originally installed by the builder and the home is only 10 years old and it is a reputable builder. I haven't had any known leaks from this shower. I think I can see some concrete backer board against the wall but I'm not even sure what to apply the grout to at this point. Any advice on whether this appears normal or how to go about re-grouting would be appreciated.

I'm also looking for tips on how to remove grout along the tile between the floor and wall without damaging the wall tile.