Ubuntu 17.04 - doesn't have a swap partition?

There's nothing wrong. In fresh installations of 17.04 we have a swap file instead of a swap partition, as mentioned in the release notes (if you upgrade rather than install afresh, you will still have a swap partition).

The swap file is located at

/swapfile

You can always check the status of swap with the command swapon --show.
Here's my output in 17.04:

$ swapon --show
NAME      TYPE SIZE  USED PRIO
/swapfile file 1.3G 62.2M   -1

You can also see if it's being used with the free command as usual

$ free -h
              total        used        free      shared  buff/cache   available
Mem:           1.9G        1.3G        152M         95M        423M        328M
Swap:          1.3G         62M        1.3G

See - working just fine.

According to the Arch wiki

There is no performance advantage to either a contiguous swap file or a partition, both are treated the same way.

It just provides a little more flexibility; it's easier to resize.

Minor note: Btrfs (alternative filesystem to ext4) does not support swap file usage, so you will need to create a swap partition yourself if you want to use Btrfs. But if you're brave enough to use Btrfs, you surely know what you're doing anyway ;)