It is possible, that a server sends a form with a field as follows, which
has no (default) value assigned.
```
<field label="foo" type="list-single" var="bar">
<option label="a"><value>a</value></option>
<option label="b"><value>b</value></option>
<option label="c"><value>c</value></option>
</field>
```
This patch fixes profanity to show that list. Before this patch
profanity showed nothing.
I stumbled over this while running the `/room config` command inside
a newly created muc on the `sure.im` XMPP server.
Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>
In accordance with Section 3.2.2.1 of XML Schema Part 2: Datatypes, the
allowable lexical representations for the xs:boolean datatype are the
strings "0" and "false" for the concept 'false' and the strings "1" and
"true" for the concept 'true'; implementations MUST support both styles
of lexical representation.
