Null check onComplete parameter for scheduleChunkLoad with gen = false

When gen = true, the parameter may be null. There is no reason it
should NPE then when gen = false.

src/main/java/ca/spottedleaf/moonrise/patches/chunk_system/scheduling/ChunkTaskScheduler.java

@@ -457,12 +457,16 @@ public void scheduleChunkLoad(final int chunkX, final int chunkZ, final boolean
         }
         this.scheduleChunkLoad(chunkX, chunkZ, ChunkStatus.EMPTY, addTicket, priority, (final ChunkAccess chunk) -> {
             if (chunk == null) {
-                onComplete.accept(null);
+                if (onComplete != null) {
+                    onComplete.accept(null);
+                }
             } else {
                 if (chunk.getPersistedStatus().isOrAfter(toStatus)) {
                     this.scheduleChunkLoad(chunkX, chunkZ, toStatus, addTicket, priority, onComplete);
                 } else {
-                    onComplete.accept(null);
+                    if (onComplete != null) {
+                        onComplete.accept(null);
+                    }
                 }
             }
         });