Daophot reader crashes on null USER

Hi,

`io.ascii.daophot.Daophot` reader crashes when USER field in the header (comment) section is empty. This happens when IRAF user is not defined when `phot` task is run.

```
#K IRAF       = NOAO/IRAFV2.16    version    %-23s     
#K USER       =                              name       %-23s     
#K HOST       = blah.blah.com         computer   %-23s
...
```

Currently, the fix is to manually modify the file to look like this:

```
#K IRAF       = NOAO/IRAFV2.16          version    %-23s     
#K USER       =  ""                               name       %-23s     
#K HOST       = blah.blah.com              computer   %-23s
...
```

Can the reader not be so sensitive and automatically replace a null user with an empty string (or something similar)?