subset or grep "NAs" from a specific column of a data frame or data table

I know that you can subset NAs from a specific column with:

subset(df,column=="")

And I also built a function integrating the former in:

subset.grep<-function(selfrom,where,q){
    if (q==""){
        result<-subset(selfrom,where=="")
    } else{
         for(i in q) 
             result<-selfrom[ grep(i, selfrom[[where]]), ]
          }
    return(result)
}

This way I can subset a data frame given a query, even when an element of the query is not present in the targeted data frame.

It works when q!="", but when q=="", it does't appear to be working properly in my function.