[Bug Report] Table column sort not handle null or undefined value correctly

### Element UI version
2.12.0

### OS/Browsers version
FF 60 / Chrome 76

### Vue version
2.6.10

### Reproduction Link
https://jsfiddle.net/14rey29c/

### Steps to reproduce
Sort table column which contain null or undefined or empty values

### What is Expected?
Rows with null or undefined or empty values should be at the end or the beginning of the table, depending on the sort direction

### What is actually happening?
Rows with null or undefined or empty values aren't sorted and seem to be at randomly position