Fix zero count issue in SPARQL return for bar charts about scholia HOT 1 OPEN

#defaultView:BarChart
# Inspired from LEGOLAS - http://abel.lis.illinois.edu/legolas/
# Shubhanshu Mishra, Vetle Torvik
SELECT
  (STR(?year) AS ?year)
  (COUNT(?work) AS ?number_of_publications)
  ?role 
WHERE {
  {
    VALUES ?year { 
      2000 2001 2002 2003 2004 2005 2006 2007 2008 2009
      2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
    }
  }
  UNION 
  {
    {
      select ?work (min(?years) as ?year) (count(?coauthors) as ?number_of_authors) ?author_number where {
        ?work (p:P50|p:P2093) ?author_statement . 
        ?author_statement ps:P50 wd:Q20984804 .
        optional { ?author_statement pq:P1545 ?author_number . }
        ?work (wdt:P50|wdt:P2093) ?coauthors . 
        ?work wdt:P577 ?dates .
        bind(str(year(?dates)) as ?years) .
      }
      group by ?work ?author_number
    }
    bind(coalesce(if(?number_of_authors = 1,
              'Solo author',
              if(xsd:integer(?author_number) = 1,
                 'First author',
                 if(xsd:integer(?author_number) = ?number_of_authors,
                    'Last author',
                    'Middle author'))), 'Unknown')
         as ?role)
   }
}
group by ?year ?role
order by ?year