Allow log max size to not be set

The max-size log opt is not valid for all logging drivers, such as
syslog. Allow the option to be removed from the boot config with:

```
kamal proxy boot_config set --log-max-size=
or
kamal proxy boot_config set --log-max-size=""
```

lib/kamal/cli/proxy.rb

@@ -37,6 +37,7 @@ class Kamal::Cli::Proxy < Kamal::Cli::Base
       ]
 
       on(KAMAL.proxy_hosts) do |host|
+        p boot_options
         execute(*KAMAL.proxy.ensure_proxy_directory)
         upload! StringIO.new(boot_options.join(" ")), KAMAL.config.proxy_options_file
       end

lib/kamal/configuration.rb

@@ -254,7 +254,7 @@ class Kamal::Configuration
   end
 
   def proxy_logging_args(max_size)
-    argumentize "--log-opt", "max-size=#{max_size}"
+    argumentize "--log-opt", "max-size=#{max_size}" if max_size.present?
   end
 
   def proxy_options_default