[json] no IntelliSense when all-optional branch matches

E.g. with 3 branches in a `"oneOf"`: ```json { "$schema": "http://json-schema.org/draft-07/schema#", "oneOf": [ { "type": "object", "properties": { "dockerFile": { "type": "string" } }, "required": [ "dockerFile" ], "additionalProperties": false }, { "type": "object", "properties": { "image": { "type": "string" } }, "required": [ "image" ], "additionalProperties": false }, { "type": "object", "properties": { }, "additionalProperties": false } ] } ``` `"dockerFile"` or `"image"` are allowed but not both at the same time. The real-life schema has several optional properties and also more required properties. I don't get proposals for `"dockerFile"` or `"image"` in an empty object:  Removing the third (here empty) branch makes it show `"dockerFile"` and `"image"` proposals.