Fixed crash, when reek reports '0 total warnings'.

```
gems/metric_fu-4.7.2/lib/metric_fu/metrics/reek/reek.rb:27:in `block in analyze': undefined method `gsub' for nil:NilClass (NoMethodError)
gems/metric_fu-4.7.2/lib/metric_fu/metrics/reek/reek.rb:25:in `map'
gems/metric_fu-4.7.2/lib/metric_fu/metrics/reek/reek.rb:25:in `analyze'
gems/metric_fu-4.7.2/lib/metric_fu/metrics/generator.rb:115:in `generate_result'
gems/metric_fu-4.7.2/lib/metric_fu/reporting/result.rb:50:in `add'
gems/metric_fu-4.7.2/lib/metric_fu/run.rb:20:in `block in measure'
gems/metric_fu-4.7.2/lib/metric_fu/run.rb:18:in `each'
gems/metric_fu-4.7.2/lib/metric_fu/run.rb:18:in `measure'
gems/metric_fu-4.7.2/lib/metric_fu/run.rb:9:in `run'
gems/metric_fu-4.7.2/lib/metric_fu/cli/helper.rb:18:in `run'
gems/metric_fu-4.7.2/lib/metric_fu/cli/client.rb:18:in `run'
gems/metric_fu-4.7.2/bin/metric_fu:9:in `<top (required)>'
bin/metric_fu:23:in `load'
bin/metric_fu:23:in `<main>'
bin/ruby_executable_hooks:15:in `eval'
bin/ruby_executable_hooks:15:in `<main>'
```

With 0 warnings the report is empty and the graph shows "No Data".