Fix error when no shared frontends are defined.

Fix the following error which occurs when no shared frontends are defined:
```
synapse/lib/synapse/haproxy.rb:560:in `generate_config': undefined method `flatten' for nil:NilClass (NoMethodError)
        from synapse/lib/synapse/haproxy.rb:534:in `update_config'
        from synapse/lib/synapse.rb:45:in `block in run'
        from synapse/lib/synapse.rb:37:in `loop'
        from synapse/lib/synapse.rb:37:in `run'
        from synapse/bin/synapse:60:in `<top (required)>'
        from synapse/binstubs/synapse:16:in `load'
        from synapse/binstubs/synapse:16:in `<main>'
```