JPL SBDB query fails to get proper diameter value

I was testing SBDB module with some test cases. When I tested 1980 PA:
```python
from astroquery.jplsbdb import SBDB
obj = SBDB.query('1980 PA', phys=True)
print(obj['phys_par']['diameter'])
```
gives ``km`` without any value. When I searched 1980 PA on SBDB, it shows ``1.0`` (km), so I am a bit wondering that it does not understand the value ``1``.

Python 3.7 on macOS 10.14.6 with astroquery 0.3.10.dev5703.

Start at the SBDB.query entry point using the reported 1980 PA example with phys=True, and inspect how the returned phys_par diameter is parsed. Compare the result with the SBDB value of 1.0 km; done means the query returns the numeric diameter rather than only the km unit.

Written by the indexing model from the issue text.