Currently running 1.12 pre, and cleared my ReplaceToBiomeName to blank, set Biomewetness to 0.0 and it's still raining in the custom biome. Current build 179 spigot 1.12 pre 5

Biome wetness. Float value from 0.0 to 1.0.
If this biome is a custom biome, and this value is set to 0, no rain will fall.
On default biomes, this won't do anything except changing the grass and leaves colors slightly.
BiomeWetness: 0.0

Unfortunately, the Minecraft client doesn't know that the wetness changed. The server doesn't (and cannot) tell the Minecraft client this. So this issue cannot be fixed without some client-side mod.