blocktrans is crashing when using percent signs when no variables were supplied

[Mikail]

When using blocktrans without supplying any variables and a translator puts a percent sign as a translation, a crash occurs.

For example, using this template snippet:

{% blocktrans context "Taxes" %}
  tax_rate * price
{% endblocktrans %}

If one translates it to:

  % imposto * preço

It will raise:

TypeError: %i format: a number is required, not dict

Where stack is:

  File "django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "django/core/handlers/base.py", line 145, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "django/core/handlers/base.py", line 143, in _get_response
    response = response.render()
  File "django/template/response.py", line 106, in render
    self.content = self.rendered_content
  File "django/template/response.py", line 83, in rendered_content
    content = template.render(context, self._request)
  File "django/template/backends/django.py", line 61, in render
    return self.template.render(context)
  File "django/template/base.py", line 171, in render
    return self._render(context)
  File "django/template/base.py", line 163, in _render
    return self.nodelist.render(context)
  File "django/template/base.py", line 937, in render
    bit = node.render_annotated(context)
  File "django/template/base.py", line 904, in render_annotated
    return self.render(context)
  File "django/template/loader_tags.py", line 150, in render
    return compiled_parent._render(context)
  File "django/template/base.py", line 163, in _render
    return self.nodelist.render(context)
  File "django/template/base.py", line 937, in render
    bit = node.render_annotated(context)
  File "django/template/base.py", line 904, in render_annotated
    return self.render(context)
  File "django/template/loader_tags.py", line 150, in render
    return compiled_parent._render(context)
  File "django/template/base.py", line 163, in _render
    return self.nodelist.render(context)
  File "django/template/base.py", line 937, in render
    bit = node.render_annotated(context)
  File "django/template/base.py", line 904, in render_annotated
    return self.render(context)
  File "django/template/loader_tags.py", line 62, in render
    result = block.nodelist.render(context)
  File "django/template/base.py", line 937, in render
    bit = node.render_annotated(context)
  File "django/template/base.py", line 904, in render_annotated
    return self.render(context)
  File "django/templatetags/i18n.py", line 161, in render
    result = result % data

The crashing line being this one: ​1e429df7488/django/templatetags/i18n.py#L161

The PO compiler doesn't check the validity of the provided translation as the flag ​python-format is not set in the PO file.

We should whether not try to format the string if there is no variable, or force the python-format flag to be set to prevent translators from breaking the templates.