652: Don't show empty field labels.

If the label for a field is empty, don't show it at all.
And don't translate it!

mediagoblin/templates/mediagoblin/utils/wtforms.html

@@ -18,7 +18,9 @@
 
 {# Generically render a field #}
 {% macro render_field_div(field) %}
-  <p class="form_field_label"><label for="{{ field.name }}">{{ _(field.label.text) }}</label></p>
+  {% if field.label.text -%}
+    <p class="form_field_label"><label for="{{ field.name }}">{{ _(field.label.text) }}</label></p>
+  {%- endif %}
   <div class="form_field_input">
     {{ field }}
     {%- if field.errors -%}