Nullability Annotations to Java Classes - SqlUtils & Model - Plugin

[ParaskP7]

Parent #2799

This issue is about adding missing nullability annotations (@Nullable & @NonNull) to as many Java-related Plugin sql-utils & model classes as possible, including fields, arguments, parameters, and return values.

• plugin model package
  • ImmutablePluginModel.java
  • PluginDirectoryModel.java
  • SitePluginModel.java
  • WPOrgPluginModel.java

FYI: Note that these model classes are associated with PluginSqlUtils.kt and PluginSqlUtilsWrapper.kt, which are Kotlin classes already.