Fix some issues identified by FindBugs

I ran FindBugs on Big Reactors and fixed some of the things it found, such as:
<ul>
<li>redundant null checks</li>
<li>dead stores (assigning something to a variable and never using it)</li>
<li>boxing and unboxing a primitive int when converting from a String</li>
</ul>
I also noticed that there are null checks for things in the part sets in MultiblockReactor. The only place where things are added to these sets checks the part to be added with instanceof, therefore null will never be added to these sets because using instanceof with null always returns false.

If I've inadvertently deleted something important, as I might have with this line if it did not have a comment, please let me know:
<code>Fluid waterFluid = FluidRegistry.WATER; // Force-load water to prevent startup crashes</code>