C4ValueArraySortStringscomp: silenced string-to-bool-conversion warning

VisualStudio gave a "performance warning" about this implicit cast. I actually don't know why, but this change makes the intention clearer anyway (since we are not returning the string contents but just whether they exist). So it's probably not bad regardless of whether the warning is weird.

http://stackoverflow.com/questions/1847860/why-is-there-a-performance-warning-on-cast-pointer-to-bool

src/script/C4ValueArray.cpp

@@ -104,7 +104,7 @@ struct C4ValueArraySortStringscomp
 	{
 		if (v1.getStr() && v2.getStr())
 			return std::strcmp(v1._getStr()->GetCStr(), v2._getStr()->GetCStr()) < 0;
-		return v2.getStr();
+		return v2.getStr() != nullptr;
 	}
 };