Don't show disabled plugins as pending upgrade

Since commit c0187ec the value of `running_version` is converted
to_string (`nil` would become the empty string). But the formatter
expected `running_version` to be `nil` if the plugin is not
running. This did not match and not running/not enabled plugins were
marked incorrectly as "pending upgrade to...".

For example:
```
$ rabbitmq-plugins list trust -q
[  ] rabbitmq_trust_store               (pending upgrade to 3.13.0+51.g9145b53)

$ rabbitmq-plugins list trust --formatter erlang -q
 #{status => running,format => normal,
  plugins =>
      [#{enabled => not_enabled,name => rabbitmq_trust_store,running => false,
         version => <<"3.13.0+51.g9145b53">>,running_version => <<>>}]}
```

Author: gomoripeti

deps/rabbitmq_cli/lib/rabbitmq/cli/formatters/plugins.ex

@@ -146,7 +146,7 @@ defmodule RabbitMQ.CLI.Formatters.Plugins do
     ]
   end
 
-  defp augment_version(%{version: version, running_version: nil}) do
+  defp augment_version(%{version: version, running: false}) do
     to_string(version)
   end