Blank nodes cause 500 response ("Error translating between RDF formats") when requesting JSON-LD

Issue: When attempting to request a JSON-LD serialisation (via "Accept:application/ld+json" header) of a turtle document which includes one or more blank nodes, a 500 error is generated. An equivalent document where the blank node is replaced with a URI-identified node returns JSON-LD just fine.

Solid Version: Running on Solid 5.2.4 (current version on https://solid.community)

Expected outcome: Valid JSON-LD should be returned whether or not blank nodes are included (serialising any blank nodes as JSON objects without "@id", according to the JSON-LD spec).

Steps to reproduce

# 1: document without bnode, requested as turtle
$ http get https://musicog.solid.community/public/without_bnode.ttl
HTTP/1.1 200 OK
Content-Type: text/turtle
# ...
<http://example.com/testSubject1> <http://example.com/testPredicate><http://example.com/testSubject2> .
<http://example.com/testSubject2> <http://example.com/testPredicate2> "Example" .

# 2: document without bnode, requested as JSON-LD
$ http get https://musicog.solid.community/public/without_bnode.ttl Accept:application/ld+json
HTTP/1.1 200 OK
Content-Type: application/ld+json; charset=utf-8
# ...
[
    {
        "@id": "http://example.com/testSubject1",
        "http://example.com/testPredicate": [
            {
                "@id": "http://example.com/testSubject2"
            }
        ]
    },
    {
        "@id": "http://example.com/testSubject2",
        "http://example.com/testPredicate2": [
            {
                "@value": "Example"
            }
        ]
    }
]


# 3: document with bnode, requested as turtle
$ http get https://musicog.solid.community/public/with-bnode.ttl
HTTP/1.1 200 OK
Content-Type: text/turtle
# ...
<http://example.com/testSubject> <http://example.com/testPredicate> [ <http://example.com/testPredicate2> "Example" ] . 

# 4: document with bnode, requested as JSON-LD -> 500 error!
$ http get https://musicog.solid.community/public/with-bnode.ttl Accept:application/ld+json
HTTP/1.1 500 Internal Server Error
Content-Type: text/plain; charset=utf-8
# ... 
Error translating between RDF formats

使用 Accept:application/ld+json 分别对 with-bnode.ttl 和 without_bnode.ttl 重现这两个请求，并比较响应。从失败的请求开始，跟踪服务器将 RDF 转换为 JSON-LD 的路径。完成的标准是：包含空白节点的文档返回有效的 JSON-LD，其中空白节点表示为不带 @id 的对象，而不是返回 500 响应。