format_duration(None) raises an exception #25
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							```
 File "C:\Users\lukas\Documents\git\timetracker\src\web\tracker\models.py", line 41, in total_duration
    return format_duration(result["duration"])
  File "C:\Users\lukas\Documents\git\timetracker\src\web\common\util\time.py", line 26, in format_duration
    duration = timedelta(seconds=duration)
TypeError: unsupported type for timedelta seconds component: NoneType
```
						
						
					
					
					Avoid raising exception on format_duration(None)
				
			
					
					Show playtime total on session list
				
			
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	Reference: lukas/timetracker#25
	
	
	
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