Raise PHPStan level for the `amp/common` package to 5

## Feature description

The Common package (`ampproject/common`) is currently sitting at and enforcing level 4 of PHPStan static analysis.

There are 2 errors that need to be fixed so that we can raise the level to 5 and enforce this new quality level in Travis CI.

---------------

_Do not alter or remove anything below. The following sections will be managed by moderators only._

## Acceptance criteria

* PHPStan does not show any detected errors when run on level 5.

## Implementation brief

* Change the level to `5` in the file `phpstan.neon.dist`.
* Run `composer analyze`.
* Fix detected errors until no more are found.

## QA testing instructions

*

## Demo

*

## Changelog entry

*

Start with phpstan.neon.dist and the current level-4 configuration, then run composer analyze to identify the two errors that block level 5. Inspect the reported locations and address each issue without changing the requested analysis scope. Done means composer analyze reports no errors at level 5 and the configuration enforces that level in Travis CI.

Written by the indexing model from the issue text.