Fix sample.lua barfing on Raw data (#3358)

* Fix for "pandoc: user error (Incorrect result type (string expected, got nil))." when the source format contains Raw data.

* Update sample.lua

data/sample.lua

@@ -173,6 +173,8 @@ end
 function RawInline(format, str)
   if format == "html" then
     return str
+  else
+    return ''
   end
 end
 
@@ -321,6 +323,8 @@ end
 function RawBlock(format, str)
   if format == "html" then
     return str
+  else
+    return ''
   end
 end