enable put salt minion proxy host to empty string

When proxy parameter was defined and host is empty string, salt is complaining with warning. With this patch when host is empty parameters are not used.

Change-Id: I11150e5f141182d5934564611d6c39b2b379b5e9

salt/files/minion.conf

 log_level: {{ minion.log.get('level', 'error') }}
 state_output: {{ minion.log.get('state_output', 'changes') }}
 
-{%- if minion.proxy is defined %}
+{%- if minion.get('proxy', {}).get('host', "") != "" %}
 proxy_host: {{ minion.proxy.host }}
 proxy_port: {{ minion.proxy.port }}
 {%- endif %}