stats: fix panic when dumping stats

[alivxxx]

What problem does this PR solve?

When dumping stats, we get panic log like:

... Panic: runtime error: invalid memory address or nil pointer dereference (PC=0x45B81A)

/usr/lib/go/src/runtime/panic.go:513
  in gopanic
/usr/lib/go/src/runtime/panic.go:82
  in panicmem
/usr/lib/go/src/runtime/signal_unix.go:390
  in sigpanic
/home/aliv/go/src/github.com/pingcap/tidb/util/chunk/chunk.go:241
  in Histogram.ConvertTo
histogram.go:159
  in Histogram.ConvertTo
dump.go:103
  in Handle.tableStatsToJSON
dump.go:64
  in Handle.DumpStatsToJSON

What is changed and how it works?

If the column is not primary key, we will load the bucket info by need, thus leaving the Bound as nil. If the column is droped, we will not delete it immediately, so this column will still be in the cache, so when dumping the stats, we will meet panic when using the Bound info.

This PR fixes this by assigning empty chunk to Bound, so it will never be nil, and this will make future code does not meet the same problem again.

Check List

Tests

• Unit test

Code changes

• Has exported function/method change

Side effects

• None

Related changes

• Need to cherry-pick to the release branch

---

This change is 

[XuHuaiyu]

LGTM

[winoros]

lgtm

[alivxxx]

/run-all-tests