Avoid doubled labels.

The null identifier for spans is the empty string, not nil.

text-crossrefs/text-crossrefs.lua

@@ -216,11 +216,11 @@ local function control_label_placement(span)
     local id = span.identifier
     if label_placement == 'end' then
       span.content:insert(pandoc.Span({}, { id = id }))
-      span.identifier = nil
+      span.identifier = ''
     elseif label_placement == 'both' then
       span.content:insert(1, pandoc.Span({}, { id = id .. '-beg' })) -- for DOCX/ODT
       span.content:insert(pandoc.Span({}, { id = id .. '-end' }))
-      span.identifier = nil
+      span.identifier = ''
     elseif label_placement ~= 'beg' then
       warning('Invalid value ' .. label_placement .. ' on attribute ' .. PLACE_LABEL_ATTR .. ': ' ..
            'shoud be “beg” (default), “end” of “both”. Using the defaults.')