Make ‘var’ attribute of a Field optional

Looking at [the spec](https://xmpp.org/extensions/xep-0004.html#protocol-field)
it seems valid not to have a `var` attribute set, at least for fields of type
`fixed` that is:

> If the element type is anything other than "fixed" (see below), it MUST
> possess a 'var' attribute that uniquely identifies the field in the context
> of the form (if it is "fixed", it MAY possess a 'var' attribute). The element
> MAY possess a 'label' attribute that defines a human-readable name for the field.

parsers/src/ecaps2.rs

@@ -94,7 +94,10 @@ fn compute_extensions(extensions: &[DataForm]) -> Result<Vec<u8>, Error> {
         ));
         bytes.push(0x1e);
         bytes.append(&mut compute_items(&extension.fields, 0x1d, |field| {
-            let mut bytes = compute_item(&field.var);
+            let mut bytes = vec![];
+            if let Some(var) = &field.var {
+                bytes.append(&mut compute_item(var));
+            }
             bytes.append(&mut compute_items(&field.values, 0x1e, |value| {
                 compute_item(value)
             }));