The XXXPinotFS.isDirectory methods should return false if the provided directory doesn't exist

This would be consistent with how Java's `File.isDirectory()` (and `File.isFile()`) work.

Currently at least `HadoopPinotFS.isDirectory()` (don't know about others) throws a `FileNotFoundException` if called on a non-existent file/directory.

Locate the XXXPinotFS.isDirectory implementations, especially HadoopPinotFS.isDirectory(), and compare their handling of missing paths with Java File.isDirectory(). Add or update tests covering nonexistent files and directories, then verify each implementation returns false rather than throwing FileNotFoundException.

Written by the indexing model from the issue text.