Corrected error in ST3

When running SublimeFiles in ST3, I had the following error:

```
Traceback (most recent call last):
  File "/Applications/Sublime Text.app/Contents/MacOS/sublime_plugin.py", line 524, in run_
    return self.run(**args)
  File "/Users/chadrien/Library/Application Support/Sublime Text 3/Packages/SublimeFiles/sublime_files.py", line 30, in run
    self.open_navigator()
  File "/Users/chadrien/Library/Application Support/Sublime Text 3/Packages/SublimeFiles/sublime_files.py", line 49, in open_navigator
    for ignore_pattern in self.ignore_list:
TypeError: 'NoneType' object is not iterable
```

Problem was simply a for…in started whithout checking the var is empty or not.