Exclude countries with no rounds from show_countries

[cimentadaj]

show_countries show countries such as Serbia and Montenegro which are to appear in ESS9 as their first round. show_country_rounds knows how to exclude rounds which are still not available (ESS9) but show_countries includes them. See the table here.

# Serbia is here
show_countries()

#>  [1] "Albania"            "Austria"            "Belgium"           
#>  [4] "Bulgaria"           "Croatia"            "Cyprus"            
#>  [7] "Czechia"            "Denmark"            "Estonia"           
#> [10] "Finland"            "France"             "Germany"           
#> [13] "Greece"             "Hungary"            "Iceland"           
#> [16] "Ireland"            "Israel"             "Italy"             
#> [19] "Kosovo"             "Latvia"             "Lithuania"         
#> [22] "Luxembourg"         "Montenegro"         "Netherlands"       
#> [25] "Norway"             "Poland"             "Portugal"          
#> [28] "Romania"            "Russian Federation" "Serbia"            
#> [31] "Slovakia"           "Slovenia"           "Spain"             
#> [34] "Sweden"             "Switzerland"        "Turkey"            
#> [37] "Ukraine"            "United Kingdom"
    
# But there no rounds. This makes sense once you look at the link from above.
show_country_rounds('Serbia')

#> numeric(0)