Don't show 'empty' metadata field

Change logic for when to show simple v advanced editor to use an
explicit flag.

cms/templates/widgets/problem-edit.html

 <%include file="metadata-edit.html" />
 <section class="problem-editor editor">
     <div class="row">
-        %if markdown != '' or data == '<problem>\n</problem>\n':
+        %if enable_markdown:
         <div class="editor-bar">
             <ul class="format-buttons">
                 <li><a href="#" class="header-button" data-tooltip="Heading 1"><span

common/lib/xmodule/xmodule/capa_module.py

@@ -703,15 +703,15 @@ class CapaDescriptor(RawDescriptor):
 
     def get_context(self):
         _context = RawDescriptor.get_context(self)
-        _context.update({'markdown': self.metadata.get('markdown', '')})
+        _context.update({'markdown': self.metadata.get('markdown', ''),
+                         'enable_markdown' : 'markdown' in self.metadata})
         return _context
 
     @property
     def editable_metadata_fields(self):
         """Remove metadata from the editable fields since it has its own editor"""
-        subset = super(CapaDescriptor, self).editable_metadata_fields
-        if 'markdown' in subset:
-            subset.remove('markdown')
+        subset = [field for field in super(CapaDescriptor,self).editable_metadata_fields
+                  if field not in ['markdown', 'empty']]
         return subset
 
 

common/lib/xmodule/xmodule/html_module.py

@@ -172,6 +172,13 @@ class HtmlDescriptor(XmlDescriptor, EditingDescriptor):
         elt.set("filename", relname)
         return elt
 
+    @property
+    def editable_metadata_fields(self):
+        """Remove metadata from the editable fields since it has its own editor"""
+        subset = [field for field in super(HtmlDescriptor,self).editable_metadata_fields
+                  if field not in ['empty']]
+        return subset
+
 
 class AboutDescriptor(HtmlDescriptor):
     """