Fix `kendalltau` + fix `nothing` for `SegmentedWindowConfig`

[JanJereczek]

Two minor fixes:

• kendalltau(x, y) requires x::Vector and y::Vector. This was previously not enforced in our code and is now the case by the use of collect()
• Passing nothing as indicator was not handled in segmentedWindowConfig, which has been corrected.

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (b1d3387) 36.42% compared to head (ed008ef) 44.84%.

Files	Patch %	Lines
src/analysis/segmented_window.jl	0.00%	5 Missing :warning:
src/change_metrics/slope.jl	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #74 +/- ## ========================================== + Coverage 36.42% 44.84% +8.42% ========================================== Files 18 17 -1 Lines 442 359 -83 ========================================== Hits 161 161 + Misses 281 198 -83 ```

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