So, what? A directory with music in it? How about a directory with directories with music in it? What if there are other non-music files in the directory? Please understand you have to be specific since everyone has different notions of what this means. For many, just a folder named "Music" is what constitutes a music directory.
For empty directories, again, it depends on what you want. You want a file that is empty in terms of bytes? In that case, just match folders with a size of 0. Note that this includes folders with other empty folders in it. If you want a folder with nothing in it (including empty folders), then you probably want to use the Sub-file/folder count instead. In such a case, then, you'll need to account for something (either another Hazel rule or an external force) to remove the empty folders within those folders.