Fix BRAVA Error on /variables endpoint

[RichardBruskiewich]

BRAVA test error: Response structure doesn't match schema:

Test: Json matches schema: /schemas/v1.2/variables.json

Error: validation - type
In element: /result/data/0/contextOfUse
Expected: array, null
Found: string
Message: instance type (string) does not match any allowed primitive type (allowed: ["array","null"])

AND

Error: validation - minItems
In element: /result/data/2/scale/validValues/categories
Minimum items: 1
Message: array is too short: must have at least 1 elements but instance has 0 elements

NOTE: This latter error may be due to the absence of data in the test database used.

Generally, the tripal/brapi module doesn't seem to implement the full set of fields covered by the variable specification. The above error simply picks up on a field which has an expected type of 'array' (other empty or missing fields are just ignored).

[guignonv]

This issue is not related to the Drupal module as the call tested by Brava on MGIS was proxied to http://www.cropontology.org/brapi/v1.