You can get the rows with NULL value fields by creating a list of possible keys and using CROSS APPLY to associate each key to each row from the original dataset, and then left-joining in the parsed JSON.

Here's a working example you should be able to execute as-is:

-- Throw together a quick and dirty CTE containing your example data
WITH OriginalValues AS (
    SELECT *
    FROM (
        VALUES ( 1, 'Nikon', '{"4e7a":["jpg","bmp","nef"],"604e":["en"]}' ),
        ( 2, 'Canon', '{"4e7a":["jpg","bmp"],"604e":["en","jp","de"]}' ),
        ( 3, 'Olympus', '{"902c":["yes"], "4e7a":["jpg","bmp"]}' )
    ) AS T ( ID, Name, Attributes )
),

-- Build a separate dataset that includes all possible 'key' values from the JSON.
PossibleKeys AS (
    SELECT DISTINCT A.[key]
    FROM OriginalValues CROSS APPLY OPENJSON( OriginalValues.Attributes ) AS A
),

-- Get the existing keys and values from the JSON, associated with the record ID
ValuesWithKeys AS (
    SELECT OriginalValues.ID, Atts.[key], Atts.Value
    FROM OriginalValues CROSS APPLY OPENJSON( OriginalValues.Attributes ) AS Atts
)

-- Join each possible 'key' value with every record in the original dataset, and then
-- left join the parsed JSON values for each ID and key
SELECT OriginalValues.ID, OriginalValues.Name, KeyList.[key], ValuesWithKeys.Value
FROM OriginalValues
CROSS APPLY PossibleKeys AS KeyList
LEFT JOIN ValuesWithKeys
    ON OriginalValues.ID = ValuesWithKeys.ID
    AND KeyList.[key] = ValuesWithKeys.[key]
ORDER BY ID, [key];

If you need to include some pre-determined key values where some of them might not exist in ANY of the JSON values stored in Attributes, you could construct a CTE (like I did to emulate your original dataset) or a temp table to provide those values instead of doing the DISTINCT selection in the PossibleKeys CTE above. If you already know what your possible key values are without having to query them out of the JSON, that would most likely be a less costly approach.

You can get the rows with NULL value fields by creating a list of possible keys and using CROSS APPLY to associate each key to each row from the original dataset, and then left-joining in the parsed JSON.

Here's a working example you should be able to execute as-is:

-- Throw together a quick and dirty CTE containing your example data
WITH OriginalValues AS (
    SELECT *
    FROM (
        VALUES ( 1, 'Nikon', '{"4e7a":["jpg","bmp","nef"],"604e":["en"]}' ),
        ( 2, 'Canon', '{"4e7a":["jpg","bmp"],"604e":["en","jp","de"]}' ),
        ( 3, 'Olympus', '{"902c":["yes"], "4e7a":["jpg","bmp"]}' )
    ) AS T ( ID, Name, Attributes )
),

-- Build a separate dataset that includes all possible 'key' values from the JSON.
PossibleKeys AS (
    SELECT DISTINCT A.[key]
    FROM OriginalValues CROSS APPLY OPENJSON( OriginalValues.Attributes ) AS A
),

-- Get the existing keys and values from the JSON, associated with the record ID
ValuesWithKeys AS (
    SELECT OriginalValues.ID, Atts.[key], Atts.Value
    FROM OriginalValues CROSS APPLY OPENJSON( OriginalValues.Attributes ) AS Atts
)

-- Join each possible 'key' value with every record in the original dataset, and
-- then left join the parsed JSON values for each ID and key
SELECT OriginalValues.ID, OriginalValues.Name, KeyList.[key], ValuesWithKeys.Value
FROM OriginalValues
CROSS APPLY PossibleKeys AS KeyList
LEFT JOIN ValuesWithKeys
    ON OriginalValues.ID = ValuesWithKeys.ID
    AND KeyList.[key] = ValuesWithKeys.[key]
ORDER BY ID, [key];

If you need to include some pre-determined key values where some of them might not exist in ANY of the JSON values stored in Attributes, you could construct a CTE (like I did to emulate your original dataset) or a temp table to provide those values instead of doing the DISTINCT selection in the PossibleKeys CTE above. If you already know what your possible key values are without having to query them out of the JSON, that would most likely be a less costly approach.