Don't remove root Tags when empty

It unnecessarily complicates things.

lib/db.coffee

@@ -333,6 +333,7 @@ fetchDevice = (id, timestamp, patterns, callback) ->
               {object: [timestamp, 0], writable: [timestamp, 0], value: [timestamp, [v, 'xsd:string']]}])
           delete device[k]
         when '_tags'
+          if v.length
             res.push([['Tags'], timestamp,
               {object: [timestamp, 1], writable: [timestamp, 0]}])
           for t in v

lib/session.coffee

@@ -698,13 +698,6 @@ rpcRequest = (sessionContext, _declarations, callback) ->
           toClear = device.set(sessionContext.deviceData, p, timestamp, {object: [timestamp, false], writable: [timestamp, true], value: [timestamp, [true, 'xsd:boolean']]}, toClear)
         else if c? and not v
           toClear = device.set(sessionContext.deviceData, p, timestamp, toClear)
-          noMoreTags = true
-          for p in sessionContext.deviceData.paths.find(['Tags', '*'], false, true)
-            if sessionContext.deviceData.attributes.has(p)
-              noMoreTags = false
-              break
-          if noMoreTags
-            toClear = device.set(sessionContext.deviceData, ['Tags'], timestamp, null, toClear)
       )
 
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