Fix NPE in PartFluidImport.java

[karmaBonfire]

This change should fix an NPE caused by fluid possibly being null. I wasn't able to reproduce a related crash on clean GT:NH install, but my custom server with latest ExtraCells2 build from this repository was crashing under weird circumstances (still investigating), with crash report pointing to this line.

---- Minecraft Crash Report ----
// Don't be sad. I'll do better next time, I promise!

Time: 8/15/22 1:50 PM
Description: Ticking GridNode

java.lang.NullPointerException: Ticking GridNode
    at extracells.part.PartFluidImport.fillToNetwork(PartFluidImport.java:164)
    at extracells.part.PartFluidImport.doWork(PartFluidImport.java:83)
    at extracells.part.PartFluidIO.tickingRequest(PartFluidIO.java:256)
    at appeng.me.cache.TickManagerCache.onUpdateTick(TickManagerCache.java:92)
    at appeng.me.GridCacheWrapper.onUpdateTick(GridCacheWrapper.java:43)
    at appeng.me.Grid.update(Grid.java:286)
    at appeng.hooks.TickHandler.onTick(TickHandler.java:349)
    at cpw.mods.fml.common.eventhandler.ASMEventHandler_569_TickHandler_onTick_TickEvent.invoke(.dynamic)
    at cpw.mods.fml.common.eventhandler.ASMEventHandler.invoke(ASMEventHandler.java:54)
    at cpw.mods.fml.common.eventhandler.EventBus.post(EventBus.java:168)
    at cpw.mods.fml.common.FMLCommonHandler.onPostServerTick(FMLCommonHandler.java:252)
    at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:860)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:683)
    at java.lang.Thread.run(Unknown Source)