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

[shrektan]

Closes #133

Example Code

con <- DBI::dbConnect(RSQLite::SQLite(), ":memory:")
sql <- 'select * from tbl where values in ({v*})'
v <- character()
glue_sql(sql, .con = con)

• Before it returns an empty SQL: <SQL>
• Now it returns a valid SQL: <SQL> select * from tbl where values in (NULL)

[jimhester]

Ok I think that makes sense, thanks!