[show] Allow system with no ports in config db run without errors (#1754

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What I did
Allow system with no ports in config db run without errors.
This is needed for modular system which should boot properly without line cards.

How I did it
Do not raise warning if ports dictionary is empty.

How to verify it
Run show interfaces status

utilities_common/cli.py

@@ -131,7 +131,7 @@ def __init__(self, db=None):
 
 
         if not self.port_dict:
-            click.echo(message="Warning: failed to retrieve PORT table from ConfigDB!", err=True)
+            click.echo(message="Configuration database contains no ports")
             self.port_dict = {}
 
         for port_name in self.port_dict: