Jumping to a unique directory when in that directory throws an error.

Steps to reproduce:

create a uniquely named directory:
```
mkdir abcdefg
```
enter directory to add entry into database:
```
cd abcdefg
```
leave directory, then jump to it (works as expected):
```
cd; j abcdefg
```

jump to the same directory again:

j abcdefg
autojump: directory 'abcdefg' not found
Try `autojump --help` for more information.

The correct behavior should be jumping back into the same directory (or doing nothing).

It's not as simple as modifying the j function because autojump returns an empty string for both highest weight match (i.e. autojump abcdefg) and missing entries (i.e. autojump ffehiofeahfieoa).