An unanalyzed and empty partitioned table with no record inserted into mysql.stats_histograms's analyze might got panicked

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

A clean partitioned table without any data inserted into it.
Make sure it has no record in `mysql.stats_histograms`(It can happen when there's a lot of DDL executed in a row).

Analyze the table.

### 2. What did you expect to see? (Required)

The analyze executes successfully.

### 3. What did you see instead (Required)

Panicked.

### 4. What is your TiDB version? (Required)

master