Consider enforcing non-empty` assertions` arrays in `annotations/test.schema.json`

[SHUBHANSHU602]

While reviewing annotations/test.schema.json, I noticed that the assertions property is defined as:
"assertions": { "type": "array", "items": { "$ref": "./assertion.schema.json" } }
However, there is currently nominItemsconstraint. This means a test of the form:

{ "instance": {}, "assertions": [] }

would be considered valid according to the schema.

From a structural perspective, a test with an empty assertions array does not appear to verify any behavior and would therefore always pass trivially. That may be acceptable if intentional, but it could also allow accidental empty tests to slip into the suite unnoticed.

Happy to raise PR !