Dungeons can generate without an entirely solid floor as I found one on one of my test worlds. I can tell this is a bug, because for each attempt to generate a dungeon, (there are 8 per chunk) the following conditions must be met:

The floor area (including under the walls) of the dungeon must be entirely solid.
The ceiling area (including over the walls) of the dungeon must be entirely solid.
The walls of the dungeon must have 2–5 openings (2-high air blocks) at the floor level.

These conditions specified are based off of information on the Minecraft Wiki. NOTE: If this is a duplicate, I searched for issues, but didn't see my issue submitted.

What I expected to happen was:
The attempt to generate the dungeon would fail as a solid floor wasn't present.

What actually happened was:
The attempt to generate the dungeon succeded, resulting in a hole in the floor. (The dungeon generated over a ravine)

How to reprduce:
1. Create a world with cheats enabled. (My level seed was: 65928447927283)
2. Switch to spectator gamemode (/gamemode sp on survival, or F3 + N on creative)
3. Search underground for dungeons, you will eventually find one without a solid floor. (If your level seed is 65928447927283, go to coordinates: x: 480 y:38 z:1159)