glue_sql() collapses zero-length DBI::SQL object into DBI::SQL("NULL")

[shrektan]

This is a missed case of #134

The real use case that hurts in reality is :

When using Oracle database, the date object must be in format like date'yyyy-mm-dd'. Thus, we need to do:

sql_dates <- DBI::SQL(sprintf("date'%s'", dates))
glue::glue_sql("select * from table where ref_date in ({sql_dates*})", .con = con)

Thus, when dates is zero-length, sql_dates would be zero-length as well, but it leads to an empty SQL string in the current version of glue.

This PR fixes this. It results a valid SQL: select * from table where ref_date in (NULL)

[jennybc]

@jimhester Do you agree this is good to go?