Allow Entity Offshore-Pump to have fluid = nil flag

[TreefrogGreaken]

Hello,

Please could you allow the fluid = "water" flag to be nil or "None" on the Offshore-Pump entity type. If you set this to nil currently, Factorio complains that the flag is missing.

Because of the new fluid mixing logic my modified pump, an "Offshore-Drain" now backfills any pipes attached to it with a dummy fluid I have had to put into the entity for it to run. Mod is in my signature if you want to look at the current entities.lua.

This wasn't a problem in 0.16 as the fluid would never escape into the pipes, as the drain has a input rather then an output.

In 0.17, as soon as you attach a pipe to the input, it locks the pipes with the dummy fluid. If the offshore pump entity was allowed to have a nil fluid type then the "offshore-drain" would work with no issues.

Think this would also solve a lot of issue when trying to make alternate offshore pumps as instead of having to modify a different entity to look like and behave like a pump to get around this fluid issue, the flag could just be made nil.

This would really save me a massive headache I'm having in trying to work around this.

Thanks