SQL & PL/SQL

I have table called NATIONAL_RARE_ECOSYSTEMS which has 1 column called TEST_COLUMN (data type: varchar2):

TEST_COLUMN
rare ecosystem
rare
0
0
(null)
(null)

what I want is a query which will add a column called NRE_SCORE which will give each row instance a score of 0 if it null.
If it is 0 then score should be 0.
If the row contains any text then score should be 1

I have written the query:

SELECT
(CASE WHEN test_column is null THEN 0
WHEN test_column = 0 THEN 0
WHEN test_column > 0 THEN 1
END) AS NRE_SCORE
FROM NATIONAL_RARE_ECOSYSTEMS;

I get the error message:

ORA-01722: invalid number
01722. 00000 - "invalid number"

I think this is because on the 2nd and 3rd line I'm trying to do arithmetic on a column which is varchar2 which I know I cant do.

How do I write a query which says: if the row contains text then give score of 1?

I'm using oracle 11g.