Add nullability annotations

the code in the "Content-Package to Feature Model Converter" should come with nullabillity annotations for method parameters and return values.

take for example https://github.com/apache/sling-org-apache-sling-feature-cpconverter/blob/master/src/main/java/org/apache/sling/feature/cpconverter/accesscontrol/DefaultAclManager.java#L142-L165:

• first checks for list of 'authorizations being null (line 147)
• on line 160 calls 'addPath', which turn calls 'areEmpty' (i.e. checks for null or empty)
• on line 164 calls 'addAclStatement', which in turn checks for null again and also calls 'areEmpty' (ie. checks for null or empty, see also SLING-9973)

if my counting is correct:

• 4 times testing if the given list is null
• 2 times testing if the given list is empty

where it could be just 1 single call to 'areEmpty' at the beginning of  the top method addStatements and return if that condition was met. i wouldn't be surprised if similar things could be found elsewhere in the module.

IMHO consistently using notnull/nullable annotations and being conscious about it helps to prevent that kind of mistakes and makes the code more robust, easier to test and to read.