Resource fields default to null instead of sensible empty values

[elharo]

In src/main/java/org/apache/maven/shared/filtering/Resource.java, lines 31–36:

All six fields default to null rather than sensible empty values. The getExtension() method in DefaultMavenResourcesFiltering handles the edge case where resource.getDirectory() might be null (line 179), but every consumer must null-check these fields. For example, Resource.getIncludes() returns null by default, not an empty list, requiring null checks at all call sites like DefaultMavenResourcesFiltering.setupScanner() at line 410.