asArray will not include null valued keys

fixes #1311

[ci skip] Перевод runtime-handling-errors.md на русский язык

Fixes #3511: Dropped `yii.allowAction()` and modified `yii.confirm()` in `yii.js` to support callbacks

[ci skip] Russian translation of runtime-url-handling.md

Добавил перевод файла runtime-url-handling.md

Fixes #2914: `ActiveForm::fieldConfig` will be merged recursively with the `$options` parameter in `ActiveForm::field()`

[skip ci] Added more missing serbian translations

Simplified ArrayTrait::fields() default implementation since it works fine w/ just field names as values

[ci skip] pt-BR start-hello

Fixes #2380: `yii\widgets\ActiveForm` will register validation js even if there are not fields inside