Can't remove ephemeral history item with [space][return]

[timbertson]

In 9fdc4f9, ephemeral history items were added.

One (subtle) feature is that this item is cleared if you type [space][return]. I want to use that feature, but it doesn't seem to work on my latest fish (v3.3.1). After doing that, the previous ephemeral item can still be recalled.

It looks like this behaviour was taken out here:

a427bf2

The behaviour and the comment describing it is removed, but there's seemingly no justification for actually changing this feature, the commit message implies it's just a crash-fix.

To reproduce:

# (command with leading space)
$  echo 1
1
# (solitary space)
$  
# (press the up arrow)
$  echo 1
1

---

$ fish --version
fish, version 3.3.1
$ uname -r
5.13.9-200.fc34.x86_64

[timbertson]

/cc @faho in case you remember a427bf2