Make all queries return None when no Simbad result is retrieved.
Add tests to verify that null responses return None.
Fix issue with extra space that breaks wildcard queries.

This fixes issue #371.  Previously, for a query on "m [0-9]" the
script line "query id wildcard  m [0-9]" would be generated, which
returns no results from Simbad.  Now, the line
"query id wildcard m [0-9]" is generated in the case of a wildcard,
which works.  This hack is a quick workaround; the fragile
_args_to_payload() function should be refactored ASAP.
Merge pull request #371 from rickyegeland/e90ac8e

Simbad query_object() with wildcard returns no results