Fix null check in fixReferences to prevent crash on null property values

The fixReferences function in jsonSchema_v2.ts would crash with
"Cannot read properties of null (reading '$ref')" when a task
definition from an extension contained null property values.

typeof null === 'object' in JavaScript, so the guard on line 31
would pass for null values, causing a recursive call into null.
The array element path (line 21) already had a null check but
the object property path was missing it.

Agent-Logs-Url: https://github.com/microsoft/vscode/sessions/97290369-e210-4404-b2b7-7c8ca4a2f84b

Co-authored-by: bryanchen-d <[email protected]>