scatter-range-scheduler cannot work well

[lhy1024]

Bug Report

1. we expect the tolerance ratio in range cluster should be 2, but it doesn't take effect.
   

   pd/server/schedulers/scatter_range.go

   Line 220 in 3f53f1e

   c.SetTolerantSizeRatio(2)

   
   

   pd/server/schedulers/utils.go

   Line 154 in 3f53f1e

   tolerantSizeRatio := cluster.GetOpts().GetTolerantSizeRatio()

2. we expect it can schedule empty region, but it can not and test can not detect
   

   pd/server/core/region.go

   Line 96 in 3f53f1e

   EmptyRegionApproximateSize = 1

   
   

   pd/server/schedulers/balance_test.go

   Lines 1103 to 1111 in 3f53f1e

   	// empty case
   	regions[49].EndKey = []byte("")
   	for _, meta := range regions {
   	leader := rand.Intn(4) % 3
   	regionInfo := core.NewRegionInfo(
   	meta,
   	meta.Peers[leader],
   	core.SetApproximateKeys(96),
   	core.SetApproximateSize(96),