Do not require args when creating a CallExpression

[zbraniecki]

While working on https://bugzilla.mozilla.org/show_bug.cgi?id=1424682 I noticed that it's not possible to create a CallExpression without passing args, even if empty.

I think it'll be pretty common in migration code to want to call FTL.CallExpression("PLATFORM") etc.

[flodolo]

I'm not sure if this is related to this patch, but when running a dry-run on all locales, I see failures on some of them (old ones, like csb, ve, ss).

The error happens on the PLATFORM() migration

Running migration bug_1424682_preferences_chrome for csb
WARNING:migrate:Missing plural categories for csb
INFO:migrate:Localization file browser/browser/preferences/preferences.ftl does not exist and it will be created
Traceback (most recent call last):
  File "/Users/flodolo/mozilla/mercurial/mozilla-unified/third_party/python/fluent/tools/migrate/migrate-l10n.py", line 120, in <module>
    dry_run=args.dry_run
  File "/Users/flodolo/mozilla/mercurial/mozilla-unified/third_party/python/fluent/tools/migrate/migrate-l10n.py", line 53, in main
    snapshot = ctx.serialize_changeset(changeset['changes'])
  File "/Users/flodolo/github/python-fluent/fluent/migrate/context.py", line 363, in serialize_changeset
    for path, snapshot in self.merge_changeset(changeset)
  File "/Users/flodolo/github/python-fluent/fluent/migrate/context.py", line 363, in <dictcomp>
    for path, snapshot in self.merge_changeset(changeset)
  File "/Users/flodolo/github/python-fluent/fluent/syntax/serializer.py", line 33, in serialize
    parts.append(self.serialize_entry(entry, state))
  File "/Users/flodolo/github/python-fluent/fluent/syntax/serializer.py", line 41, in serialize_entry
    return serialize_message(entry)
  File "/Users/flodolo/github/python-fluent/fluent/syntax/serializer.py", line 104, in serialize_message
    parts.append(serialize_attribute(attribute))
  File "/Users/flodolo/github/python-fluent/fluent/syntax/serializer.py", line 114, in serialize_attribute
    indent(serialize_value(attribute.value))
  File "/Users/flodolo/github/python-fluent/fluent/syntax/serializer.py", line 122, in serialize_value
    content = serialize_pattern(pattern)
  File "/Users/flodolo/github/python-fluent/fluent/syntax/serializer.py", line 129, in serialize_pattern
    for elem in pattern.elements
  File "/Users/flodolo/github/python-fluent/fluent/syntax/serializer.py", line 137, in serialize_element
    return serialize_placeable(element)
  File "/Users/flodolo/github/python-fluent/fluent/syntax/serializer.py", line 155, in serialize_placeable
    return "{{ {}}}".format(serialize_select_expression(expr))
  File "/Users/flodolo/github/python-fluent/fluent/syntax/serializer.py", line 209, in serialize_select_expression
    parts.append(serialize_variant(variant))
  File "/Users/flodolo/github/python-fluent/fluent/syntax/serializer.py", line 220, in serialize_variant
    indent(serialize_value(variant.value))
  File "/Users/flodolo/github/python-fluent/fluent/syntax/serializer.py", line 119, in serialize_value
    multi = contain_new_line(pattern.elements)
  File "/Users/flodolo/github/python-fluent/fluent/syntax/serializer.py", line 14, in contain_new_line
    if isinstance(elem, ast.TextElement) and "\n" in elem.value
TypeError: argument of type 'NoneType' is not iterable

[Pike]

csb doesn't have prefWindow.title. There seems to be a problem propagating missing strings up to missing messages.

I propose that in this case, we shouldn't try to resurrect the message with partial platform support.

Different bug, though.

[stasm]

lgtm. Please land a corresponding change to fluent-syntax in JS to make sure args is always an array.

[stasm]

This can be written more succinctly as args or []. It is also the convention used by other classes in this file.

[stasm]

csb doesn't have prefWindow.title. There seems to be a problem propagating missing strings up to missing messages.

There's bug 1321271 about supporting partial migrations. I think it's a big feature which is out of the scope of the upcoming L20n milestone. I submitted #44 to prevent the error @flodolo ran into by explicitly never allowing partial migrations for now.