Don't fail when query_row() is called with an empty SELECT.

src/lua/internal/sysbench.sql.lua

@@ -299,7 +299,7 @@ end
 function connection_methods.query_row(self, query)
    local rs = self:query(query)
 
-   if rs == nil then
+   if rs == nil or rs.nrows == 0 then
       return nil
    end
 

tests/include/api_sql_common.sh

@@ -210,13 +210,26 @@ sysbench $SB_ARGS run
 ########################################################################
 cat >$CRAMTMP/api_sql.lua <<EOF
 c = sysbench.sql.driver():connect()
-c:query("CREATE TABLE IF NOT EXISTS t1(a INT)")
+c:query("CREATE TABLE t1(a INT)")
 c:bulk_insert_init("INSERT INTO t1 VALUES")
 c:bulk_insert_next("(1)")
 c:bulk_insert_done()
 e,m = pcall(function () c:bulk_insert_next("(2)") end)
 print(m)
 c:bulk_insert_done()
+c:query("DROP TABLE t1")
+EOF
+
+sysbench $SB_ARGS
+
+########################################################################
+# query_row() with an empty result set
+########################################################################
+cat >$CRAMTMP/api_sql.lua <<EOF
+c = sysbench.sql.driver():connect()
+c:query("CREATE TABLE t1(a INT)")
+print(c:query_row("SELECT * FROM t1"))
+c:query("DROP TABLE t1")
 EOF
 
 sysbench $SB_ARGS

tests/t/api_sql_mysql.t

@@ -114,3 +114,4 @@ SQL Lua API + MySQL tests
   10
   ALERT: attempt to call bulk_insert_next() before bulk_insert_init()
   */api_sql.lua:*: db_bulk_insert_next() failed (glob)
+  nil

tests/t/api_sql_pgsql.t

@@ -120,3 +120,4 @@ SQL Lua API + PostgreSQL tests
   10
   ALERT: attempt to call bulk_insert_next() before bulk_insert_init()
   */api_sql.lua:*: db_bulk_insert_next() failed (glob)
+  nil