Sector -2 0 fix (#322)

* Added a battle mission to Sector (-2,0) as otherwise no core would be spawned

* Fixed a crash which occurred during test execution...

... and could maybe occur during any startup with bad timing

* Added a new class for sector tests ...

... and added a test which makes sure that any predefined sector defines
a spawn point

* Added fix after applying test driven bugfixing

core/src/io/anuke/mindustry/ai/BlockIndexer.java

@@ -247,7 +247,7 @@ public class BlockIndexer{
         for(int x = quadrantX * structQuadrantSize; x < world.width() && x < (quadrantX + 1) * structQuadrantSize; x++){
             for(int y = quadrantY * structQuadrantSize; y < world.height() && y < (quadrantY + 1) * structQuadrantSize; y++){
                 Tile result = world.tile(x, y);
-                if(result.block().drops == null || !scanOres.contains(result.block().drops.item)) continue;
+                if( result == null || result.block().drops == null || !scanOres.contains(result.block().drops.item)) continue;
 
                 itemSet.add(result.block().drops.item);
             }

core/src/io/anuke/mindustry/maps/SectorPresets.java

@@ -56,7 +56,8 @@ public class SectorPresets{
                 Missions.blockRecipe(ProductionBlocks.waterExtractor),
                 new ContentMission(Items.biomatter),
                 Missions.blockRecipe(CraftingBlocks.biomatterCompressor),
-                new ContentMission(Liquids.oil)
+                new ContentMission(Liquids.oil),
+                new BattleMission()
             ),
             Array.with(Items.copper, Items.lead, Items.coal, Items.titanium)));
     }
@@ -69,6 +70,8 @@ public class SectorPresets{
         return presets.get(x, y);
     }
 
+    public GridMap<SectorPreset> getPresets() { return presets; }
+
     private void add(SectorPreset preset){
         presets.put(preset.x, preset.y, preset);
         orePresets.put(preset.x, preset.y, preset.ores);