aggregate_labels cannot handle non-sequence typed filter values

[tstadel]

Describe the bug
aggregate_labels cannot handle non-sequence typed filter values:

labels = [
            Label(
                id="standard",
                query="question",
                answer=Answer(answer="answer1", offsets_in_document=[Span(start=12, end=18)]),
                document=Document(content="some", id="123"),
                is_correct_answer=True,
                is_correct_document=True,
                origin="gold-label",
                filters={"name": 123},
            )]
aggregate_labels(labels)

results in:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/tstad/git/haystack/haystack/utils/labels.py", line 78, in aggregate_labels
    label_filter_keys = [f"{k}={''.join(v)}" for k, v in l.filters.items()] if l.filters else []
  File "/home/tstad/git/haystack/haystack/utils/labels.py", line 78, in <listcomp>
    label_filter_keys = [f"{k}={''.join(v)}" for k, v in l.filters.items()] if l.filters else []
TypeError: can only join an iterable

Expected behavior
Labels with non-sequence typed filter values can be aggregated.

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