regression: ubiquity/hide_slideshow true; crashes installer

[erelson]

My preseed file, if it includes `ubiquity/hide_slideshow true` option, causes the installer to crash.

With Ubuntu 14.04, I've used this param successfully for preseeded installs. With 20.04 (and also 18.04; so maybe 16.04 as well) preseed installs, I've had to leave this parameter out of my preseed files.

Here's the traceback off my USB installer, afterwards, from writable/install-logs-DATE/log/installer (Also attached are the full set of logs):

```
Exception in GTK frontend (invoking crash handler):
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/gi/overrides/GLib.py", line 664, in <lambda>
    func_fdtransform = lambda _, cond, *data: callback(channel, cond, *data)
  File "/usr/lib/ubiquity/ubiquity/frontend/gtk_ui.py", line 1618, in watch_debconf_fd_helper
    return callback(source, debconf_condition)
  File "/usr/lib/ubiquity/ubiquity/filteredcommand.py", line 267, in process_input
    self.frontend.debconffilter_done(self)
  File "/usr/lib/ubiquity/ubiquity/frontend/gtk_ui.py", line 1668, in debconffilter_done
    self.find_next_step(dbfilter.__module__)
  File "/usr/lib/ubiquity/ubiquity/frontend/gtk_ui.py", line 1772, in find_next_step
    self.maybe_start_installing()
  File "/usr/lib/ubiquity/ubiquity/frontend/gtk_ui.py", line 1716, in maybe_start_installing
    self.parallel_db = DebconfCommunicator('ubiquity',
  File "/usr/lib/ubiquity/ubiquity/debconfcommunicator.py", line 35, in __init__
    debconf.Debconf.__init__(self, title=title,
  File "/usr/lib/python3/dist-packages/debconf.py", line 71, in __init__
    self.setUp(title)
  File "/usr/lib/python3/dist-packages/debconf.py", line 74, in setUp
    self.version = self.version(2)
  File "/usr/lib/python3/dist-packages/debconf.py", line 83, in <lambda>
    lambda *args, **kw: self.command(command, *args, **kw))
  File "/usr/lib/python3/dist-packages/debconf.py", line 104, in command
    status = int(status)
ValueError: invalid literal for int() with base 10: ''
```

A few other references I've seen to this parameter:
- Initial feature request/release circa Ubuntu 10.10 that included it: https://bugs.launchpad.net/ubuntu/+source/ubiquity/+bug/690912
- A report for a separate issue in Xubuntu 18.04, with a warning message about the parameter: https://bugs.launchpad.net/ubuntu/+source/ubiquity/+bug/1800199