Resource fields default to null instead of sensible empty values

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.

Start with the six field declarations in src/main/java/org/apache/maven/shared/filtering/Resource.java, then inspect getExtension() in DefaultMavenResourcesFiltering and setupScanner() around line 410 to understand current null handling. Confirm the intended empty defaults for each field and verify that the affected consumers no longer need null values.

Written by the indexing model from the issue text.