SDSS.get_images() fails for some images

Example failure:

``` python
In [1]: from astroquery.sdss import SDSS

In [2]: SDSS.get_images(run=7760, camcol=4, field=33)
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
in ()
----> 1 SDSS.get_images(run=7760, camcol=4, field=33)

/usr/lib/python3.4/site-packages/astroquery/sdss/core.py in get_images(self, coordinates, radius, matches, run, rerun, camcol, field, band, timeout, cache)
679 camcol=camcol, field=field,
680 band=band, timeout=timeout,
--> 681 get_query_payload=False)
682
683 return [obj.get_fits() for obj in readable_objs]

/usr/lib/python3.4/site-packages/astroquery/sdss/core.py in get_images_async(self, coordinates, radius, matches, run, rerun, camcol, field, band, timeout, get_query_payload, cache)
643
644 if not isinstance(matches, Table):
--> 645 raise ValueError
646
647 results = []

ValueError:

In [3]:
```

The image, however, does exist: http://data.sdss3.org/fields/runCamcolField?field=33&camcol=4&run=7760

The error seems to be related to SDSS rather than astroquery. Submitting the following SQL query (taken from `request_payload`) directly through the [SkyServer](http://skyserver.sdss.org/dr12/en/tools/search/sql.aspx) will result in an empty table:

``` sql
SELECT DISTINCT p.run,p.rerun,p.camcol,p.field FROM PhotoObjAll AS p WHERE (p.run=7760 AND p.camcol=4 AND p.field=33 AND p.rerun=301)
```

I am not sure why the query fails -- maybe the field doesn't contain any primary or secondary objects?
Anyway, doing a query is redundant when using SDSS.get_images() with specified run, camcol and field, so maybe the method should be changed to bypass the query in this case?

Start in astroquery/sdss/core.py at get_images() and get_images_async(), then reproduce the run=7760, camcol=4, field=33 example and inspect the request_payload SQL. Compare the empty SkyServer result with the field page; done means get_images() returns the existing FITS image when those identifiers are supplied.

Written by the indexing model from the issue text.