Simplify `RuntimeDefinition::class` by normalizing `RuntimeDefinition::$explicitClasses`

[sjokkateer]

Q	A
Documentation	no
Bugfix	no
BC Break	no
New Feature	no
RFC	no
QA	no

Description

This PR might be considered a minor refactor/improvement.

Currently in RuntimeDefinition::__construct() a test for truthiness of $explicitClasses is done. This means that RuntimeDefinition::setExplicitClasses() is only called whenever the constructor argument$explicitClasses is not null and not an empty array. This means that after construction, RuntimeDefinition::$explicitClasses can be null, and hence the tests for truthiness throughout some of the methods is required to account for that. This pull request normalizes the null value to an empty array in the constructor, simplifying some methods and ensuring that users do not have to deal with tests for null in code in case they extend the class.

[Ocramius]

Thanks @sjokkateer!