Check pqntuples > 0 in getValue. Fixes #12973 (#12974)

src/db_postgres.nim

@@ -441,17 +441,25 @@ proc getValue*(db: DbConn, query: SqlQuery,
   ## executes the query and returns the first column of the first row of the
   ## result dataset. Returns "" if the dataset contains no rows or the database
   ## value is NULL.
-  var x = pqgetvalue(setupQuery(db, query, args), 0, 0)
-  result = if isNil(x): "" else: $x
+  var res = setupQuery(db, query, args)
+  if pqntuples(res) > 0:
+    var x = pqgetvalue(res, 0, 0)
+    result = if isNil(x): "" else: $x
+  else:
+    result = ""
 
 proc getValue*(db: DbConn, stmtName: SqlPrepared,
                args: varargs[string, `$`]): string {.
                tags: [ReadDbEffect].} =
   ## executes the query and returns the first column of the first row of the
   ## result dataset. Returns "" if the dataset contains no rows or the database
   ## value is NULL.
-  var x = pqgetvalue(setupQuery(db, stmtName, args), 0, 0)
-  result = if isNil(x): "" else: $x
+  var res = setupQuery(db, stmtName, args)
+  if pqntuples(res) > 0:
+    var x = pqgetvalue(res, 0, 0)
+    result = if isNil(x): "" else: $x
+  else:
+    result = ""
 
 proc tryInsertID*(db: DbConn, query: SqlQuery,
                   args: varargs[string, `$`]): int64 {.