Fix `rabbitmqctl status` when the disk_free cannot be determined

By adjusting RabbitMQ.CLI.InformationUnit to no longer attempt to
convert non-numeric values

deps/rabbitmq_cli/lib/rabbitmq/cli/information_unit.ex

@@ -30,14 +30,18 @@ defmodule RabbitMQ.CLI.InformationUnit do
     :rabbit_resource_monitor_misc.parse_information_unit(val)
   end
 
-  def convert(bytes, "bytes") do
+  def convert(bytes, "bytes") when is_number(bytes) do
     bytes
   end
 
-  def convert(bytes, unit) do
+  def convert(bytes, unit) when is_number(bytes) do
     do_convert(bytes, String.downcase(unit))
   end
 
+  def convert(:unknown, _) do
+    :unknown
+  end
+
   def known_unit?(val) do
     MapSet.member?(known_units(), String.downcase(val))
   end