fix megaraid_virtual_drives fact return value

Resolves this test failure:
  1) megaraid_virtual_drives on linux no adapters should be nil
     Failure/Error: Facter.fact(:megaraid_virtual_drives).value.should be_nil
       expected: nil
            got: ""

lib/facter/megaraid_virtual_drives.rb

@@ -29,6 +29,7 @@
       end
     end
 
+    next nil if vds.empty?
     vds.sort.join(',')
   end
 end