`SentenceTransformersDiversityRanker` incorrectly replaces `lambda_threshold=0.0` with `0.5`

[abdokaseb]

Describe the bug
SentenceTransformersDiversityRanker incorrectly replaces lambda_threshold=0.0 with 0.5 because of short-circuit evaluation:

self.lambda_threshold = lambda_threshold or 0.5
# BUG: 0.0 is falsy, so it becomes 0.5

This silently changes the intended behavior when users explicitly set lambda_threshold=0.0.

Expected behavior
Passing lambda_threshold=0.0 should preserve 0.0.

To Reproduce

from haystack.components.rankers import SentenceTransformersDiversityRanker

ranker = SentenceTransformersDiversityRanker(lambda_threshold=0.0)
print(f"Expected: 0.0, Actual: {ranker.lambda_threshold}")
# Expected: 0.0, Actual: 0.5

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System:
The error will be in all OSs and versions