Fix #82: Make cookie duration nullable

[rustamwin]

Q	A
Is bugfix?	✔️/❌
New feature?	✔️/❌
Breaks BC?	✔️/❌
Fixed issues	#82

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (08ed375) 88.01% compared to head (f3582cc) 87.31%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #83 +/- ## ============================================ - Coverage 88.01% 87.31% -0.71% - Complexity 95 97 +2 ============================================ Files 13 13 Lines 267 268 +1 ============================================ - Hits 235 234 -1 - Misses 32 34 +2 ``` | [Files](https://app.codecov.io/gh/yiisoft/user/pull/83?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft) | Coverage Δ | | |---|---|---| | [src/Login/Cookie/CookieLogin.php](https://app.codecov.io/gh/yiisoft/user/pull/83?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL0xvZ2luL0Nvb2tpZS9Db29raWVMb2dpbi5waHA=) | `100.00% <100.00%> (ø)` | | | [src/Login/Cookie/CookieLoginMiddleware.php](https://app.codecov.io/gh/yiisoft/user/pull/83?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL0xvZ2luL0Nvb2tpZS9Db29raWVMb2dpbk1pZGRsZXdhcmUucGhw) | `100.00% <100.00%> (ø)` | | | [config/di-web.php](https://app.codecov.io/gh/yiisoft/user/pull/83?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-Y29uZmlnL2RpLXdlYi5waHA=) | `0.00% <0.00%> (ø)` | |

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