Fix PHPStan errors for level 3

[peter-gribanov]

Line	BaseSpecification.php
41	Method Happyr\DoctrineSpecification\BaseSpecification::getFilter() should return string but empty return statement found.
63	Method Happyr\DoctrineSpecification\BaseSpecification::getSpec() should return
	Happyr\DoctrineSpecification\Filter\Filter|Happyr\DoctrineSpecification\Query\QueryModifier but empty return statement found.

Line	Spec.php
121	Method Happyr\DoctrineSpecification\Spec::andX() should return Happyr\DoctrineSpecification\Logic\AndX but returns object.
132	Method Happyr\DoctrineSpecification\Spec::orX() should return Happyr\DoctrineSpecification\Logic\OrX but returns object.

Line	Specification/Having.php
52	Method Happyr\DoctrineSpecification\Specification\Having::getFilter() should return string but empty return statement found.

Line	EntitySpecificationRepositoryTrait.php (in context of class Happyr\DoctrineSpecification\EntitySpecificationRepository)
162	Return type (Generator&iterable) of method Happyr\DoctrineSpecification\EntitySpecificationRepository::iterate() should be compatible with return type (array) of method Happyr\DoctrineSpecification\EntitySpecificationRepositoryInterface::iterate()