I tested it in a soulsand valley and made a 9x9 chunk cleared area from bedrock floor to bedrock ceiling, then replaced the whole floor with magma blocks using commands and I got no spawn whatsoever. The moment I replaced the floor by any other solid spawnable block, ghasts (and other mobs) started spawning instantly.
Ziel
I tested it in a soulsand valley and made a 9x9 chunk cleared area from bedrock floor to bedrock ceiling, then replaced the whole floor with magma blocks using commands and I got no spawn whatsoever. The moment I replaced the floor by any other solid spawnable block, ghasts (and other mobs) started spawning instantly.