Convert Character Field To Numeric

I have one table and this field is character field with save data in below.

Bonus_table->bonus_amt_field. Char(20)
======================================
Record information
0
Null
Blank
3
4
Null
Blank

if i want to convert this character field => change to numeric field to display ,how to handle "Blank" and "null" record?

The result expect:
0
0
0
3
4
0
0

I try this query but wrong message :

select cast(convert(numeric,3)bonus_amt) as bonus_amt
from test