.index files are regenerated periodically for bnd projects
that aren't loaded into the eclipse workspace.

This fixes an issue where:
1) clean a project
2) open eclipse
3) execute a resolve --- it has an error because the project
has no .index and won't generate one since there's no .jar
file to index.
4) build outside of eclipse
5) Even though you rebuilt, the .index file isn't getting generated.

Now, the BndWorkspaceProjectIndexer will periodically
look for missing or outdated .index files.
Adding some null checks