Improve consistency of distance_of_time_in_words method

this commit improve the consistency of the method by:
- ensure that we always return the distance of time in words #=> none
- ensure that we don't get a NoMethodError when the from_time param is nil

### Example

If we need
```ruby
def last_time_entry(time)
  if time
   distance_of_time_in_words(time)
  else
    "never"
  end
end
```

we can do instead:

```YAML
# In locale
datetime:
  distance_in_words:
    none: never
```

```ruby
# In code
def last_time_entry(time)
   distance_of_time_in_words(time)
end
```