Skip notify when blank message

# Why?
I want to notify only deploy success or failed

# Usage
When only explicitly set `nil`, `false` or blank string, skip notify

# Example
```ruby
# skip deploy start notification
set :chatwork_deploy_started_message, false

# skip deploy finished notification
set :chatwork_deploy_finished_message, nil

# skip deploy failed notification
set :chatwork_deploy_failed_message, ""
```