Fix faa845398f: Crash when loading tile layout with no building sprites.

Tile layouts are permitted to have no building sprites, therefore the test for an empty sequence of sprites is not valid.

src/newgrf_commons.cpp

@@ -583,7 +583,6 @@ void NewGRFSpriteLayout::Allocate(uint num_sprites)
  */
 void NewGRFSpriteLayout::AllocateRegisters()
 {
-	assert(!this->seq.empty());
 	assert(this->registers.empty());
 
 	this->registers.resize(1 + this->seq.size(), {}); // 1 for the ground sprite