Add ability to use `Config` without params

[vjik]

Q	A
Is bugfix?	❌
New feature?	✔️
Breaks BC?	❌
Fixed issues	-

[codecov[bot]]

Codecov Report

Base: 99.57% // Head: 99.57% // Increases project coverage by +0.00% :tada:

Coverage data is based on head (c9e860d) compared to base (8cc9c42). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #131 +/- ## ========================================= Coverage 99.57% 99.57% - Complexity 270 272 +2 ========================================= Files 22 22 Lines 705 706 +1 ========================================= + Hits 702 703 +1 Misses 3 3 ``` | [Impacted Files](https://codecov.io/gh/yiisoft/config/pull/131?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft) | Coverage Δ | | |---|---|---| | [src/Config.php](https://codecov.io/gh/yiisoft/config/pull/131?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL0NvbmZpZy5waHA=) | `100.00% <100.00%> (ø)` | | 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=yiisoft). 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=yiisoft)

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