fix(apply-tags-from-csv): guard against null return from Path.getFileName()

Path.getFileName() returns null for filesystem root paths; skip such
paths instead of dereferencing the result (fixes SpotBugs NP_NULL_ON_SOME_PATH_FROM_RETURN_VALUE).

Author: leogalambos

src/main/java/org/egothor/methodatlas/ApplyTagsFromCsvEngine.java

@@ -317,7 +317,11 @@ private static Map<Path, List<MethodKey>> buildSourceIndex(List<Path> roots,
             }
             try (java.util.stream.Stream<Path> stream = Files.walk(root)) {
                 for (Path path : (Iterable<Path>) stream.filter(Files::isRegularFile)::iterator) {
-                    String name = path.getFileName().toString();
+                    Path fileNamePath = path.getFileName();
+                    if (fileNamePath == null) {
+                        continue;
+                    }
+                    String name = fileNamePath.toString();
                     if (fileSuffixes.stream().noneMatch(name::endsWith)) {
                         continue;
                     }