#46 Fix: handle non observation in DuplicateDetector

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.07 :warning:

Comparison is base (11712bf) 92.95% compared to head (0092735) 92.88%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #47 +/- ## ============================================ - Coverage 92.95% 92.88% -0.07% + Complexity 1899 1897 -2 ============================================ Files 124 124 Lines 4227 4233 +6 Branches 602 603 +1 ============================================ + Hits 3929 3932 +3 - Misses 162 163 +1 - Partials 136 138 +2 ``` | [Impacted Files](https://codecov.io/gh/open-coap/java-coap/pull/47?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-coap) | Coverage Δ | | |---|---|---| | [.../mbed/coap/server/messaging/DuplicateDetector.java](https://codecov.io/gh/open-coap/java-coap/pull/47?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-coap#diff-Y29hcC1jb3JlL3NyYy9tYWluL2phdmEvY29tL21iZWQvY29hcC9zZXJ2ZXIvbWVzc2FnaW5nL0R1cGxpY2F0ZURldGVjdG9yLmphdmE=) | `100.00% <100.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://codecov.io/gh/open-coap/java-coap/pull/47/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-coap) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-coap). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-coap)

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