PostgreSQL conditional join - if column is not NULL

Question

I have a table "temp"

    author  |  title  |  bibkey  | Data
    -----------------------------------
    John    | JsPaper | John2008 |  65
    Kate    | KsPaper |          |  60
            |         | Data2015 |  80

From this I want to produce two tables, a 'sample_table' and a 'ref_table' like so:

sample_table:

    sample_id|ref_id| data
    --------------------------
        1    |  1   |  65
        2    |  2   |  60
        3    |  3   |  80

ref_table:

    ref_id  | author  |  title  | bibkey 
    --------------------------------------
       1    |  John   | JsPaper | John2008
       2    |  Kate   | KsPaper |         
       3    |         |         | Data2015

I've created both tables

    CREATE TABLE ref_table (                CREATE TABLE sample_table (
        ref_id serial PRIMARY KEY,             sample_id serial PRIMARY KEY,
        author text,                           ref_id integer REFERENCES ref_table(ref_id),
        title text,                            data numeric
        bibkey text                         );
        );

And inserted the unique author,title,bibkey rows into the reference table as above. What I want to do now is do the join for the sample_table to get the ref_id's. For my insert statement i currently have:

    INSERT INTO sample_table (
            ref_id,data
        )
        SELECT ref.ref_id, t.data
        FROM
            temp t
            LEFT JOIN
            ref_table ref ON COALESCE(ref.author,'00000') = COALESCE(t.author,'00000')
            AND COALESCE(ref.title,'00000') = COALESCE(t.title,'00000')
            AND COALESCE(ref.bibkey,'00000') = COALESCE(t.bibkey,'00000');

However i really want to have a conditional statement in the join, rather than all 3 like I have:

1. IF a bibkey exists for that row, I know it is unique, and join only on that.
2. If bibkey is NULL, then join on both author and title for the unique pair, and not bibkey.

Is this possible?