* composer: put require- together
* add phpstan
* add phpstan to travis
* phpstan fixes
* phpstan fixes
* phpstan fixes
* drop null defaults since every property is null by default
* DescriptionFactory: fix parse nullable type
* travis: add phpstan max
* travis: fix phpstan config
The array of tags in the docblock constructor should only contain tags.
Some of the factory methods of the tags are returning a null when the tag could not be created.
This causes issues during parsing. By filtering these null values in the factory this use is resolved.