Ensure bufr table is a file in addLookupFile()

Starting in Java 25, the exsits method on File will return true for an
empty string (which is interperted as the current user directory).

Author: lesserwhirls

bufr/src/main/java/ucar/nc2/iosp/bufr/tables/BufrTables.java

@@ -140,6 +140,8 @@ public static synchronized void addLookupFile(String filename) throws FileNotFou
     if (lookups == null)
       lookups = new ArrayList<>();
     File f = new File(filename);
+    if (f.isDirectory())
+      throw new FileNotFoundException(filename + " is a directory");
     if (!f.exists())
       throw new FileNotFoundException(filename + " not found");
     lookups.add(filename);