Settings hierarchy overrides empty values

If you have an `xlwings.conf` sheet with all settings in there, empty values get overridden if you have them specified further down in the hierarchy (e.g. in a user config file). For example:

sheet
```
Conda Path | (empty)
```

user config file
```
"Conda Path","C:\some\path" 
```

Will use `C:\some\path` instead of using the empty value as it should.