Clarification on UNION ALL of JSONB_EACH result

This function used to merge 2 JSONB and delete certain key when the value is null, refer to this question.

CREATE FUNCTION jsonb_merge(JSONB, JSONB) 
RETURNS JSONB AS $$
WITH json_union AS (
    SELECT * FROM JSONB_EACH($1)
    UNION ALL
    SELECT * FROM JSONB_EACH($2)
) SELECT JSON_OBJECT_AGG(key, value)::JSONB
     FROM json_union
     WHERE key NOT IN (SELECT key FROM json_union WHERE value ='null');
$$ LANGUAGE SQL;

First question, why value = 'null' works, but value IS NULL is not?

Second question, is there any bad reason to use WHERE value <> 'null' instead of WHERE key NOT IN (SELECT key FROM ...) ?

CREATE FUNCTION jsonb_merge(JSONB, JSONB) 
RETURNS JSONB AS $$
WITH json_union AS (
    SELECT * FROM JSONB_EACH($1)
    UNION ALL
    SELECT * FROM JSONB_EACH($2)
) SELECT JSON_OBJECT_AGG(key, value)::JSONB
     FROM json_union
     WHERE value <> 'null';
$$ LANGUAGE SQL;

Third question, why this function also incorrect (always returns NULL)? Is it because UNION ALL change all resulting record into TEXT?

CREATE FUNCTION jsonb_merge(JSONB, JSONB) 
RETURNS JSONB AS $$
WITH json_union AS (
    SELECT * FROM JSONB_EACH($1)
    UNION ALL
    SELECT * FROM JSONB_EACH($2)
) SELECT JSON_OBJECT_AGG(key, value)::JSONB
     FROM json_union
     WHERE value <> (NULL::JSONB)
$$ LANGUAGE SQL;

-- check function:
SELECT COALESCE(jsonb_merge('{}','{"b":null}'),'{"x":"its empty"}'::JSONB) FROM xxx;
      coalesce      
--------------------
 {"x": "its empty"}