Avoid undefined class name

Author: myovchev

modules/@apostrophecms/area/views/area.html

@@ -1,7 +1,7 @@
 {# area needs its own copy of the widget options as #}
 {# JSON, for adding new widgets #}
 
-<div class="apos-area{{ (' ' + data._with.className) or '' }}" {%- if data.canEdit %} data-apos-area-newly-editable data-doc-id='{{ data.area._docId | jsonAttribute({ single: true }) }}' data-field-id='{{ data.field._id | jsonAttribute({ single: true }) }}' data-options='{{ apos.util.omit(data.options, 'area') | jsonAttribute({ single: true }) }}' data='{{ data.area | jsonAttribute({ single: true }) }}' data-choices='{{ data.choices | jsonAttribute({ single: true }) }}'{% endif %}{%- if data._with.style %} style="{{ data._with.style }}"{% endif %}>
+<div class="apos-area{{ (' ' + data._with.className) if  data._with.className }}" {%- if data.canEdit %} data-apos-area-newly-editable data-doc-id='{{ data.area._docId | jsonAttribute({ single: true }) }}' data-field-id='{{ data.field._id | jsonAttribute({ single: true }) }}' data-options='{{ apos.util.omit(data.options, 'area') | jsonAttribute({ single: true }) }}' data='{{ data.area | jsonAttribute({ single: true }) }}' data-choices='{{ data.choices | jsonAttribute({ single: true }) }}'{% endif %}{%- if data._with.style %} style="{{ data._with.style }}"{% endif %}>
   {%- for item in data.area.items -%}
     {%- set widgetOptions = data.options.widgets[item.type] or {} -%}
     {%- if data.options.widgetTemplate -%}