[spaces] missing optional parameter disabledFeatures results in internal error

[psanz-estc]

Kibana version: 7.2

Elasticsearch version: 7.2

Describe the bug:
When creating a space via Kibana Space API, if disabledFeatures is not defined, the space is created properly, but ML and stack monitoring tabs won’t work…

According to the doc, disabledFeatures is not required so it should not have any impact on the behavior:
disabledFeatures (string array) Optional list of features that are disabled for the space. Use the Features API to get a list of available features IDs.

Steps to reproduce:

1. Create the space via API

curl -X POST  -u elastic:password https://192.168.100.100/api/spaces/space -H 'kbn-xsrf: true' -H 'Content-Type: application/json' -d'
{
"id": "marketing",
"name": "Marketing",
"description" : "This is the Marketing Space",
"color": "#aabbcc",
"initials": "MK"
}'

2. In the browser, check the monitoring site: https://192.168.100.100/s/marketing/app/monitoring

3. Kibana returns: {"statusCode":500,"error":"Internal Server Error","message":"An internal server error occurred"}

Expected behavior:
Since disabledFeatures is optinal, space is supposed to work as designed even if the field is not there.

Any additional context:
As a workaround, the spaces can be created with disabledFeatures set to an empty array and it works fine

[elasticmachine]

Pinging @elastic/kibana-security