Add check for empty output from imgsum

In some cases like if no photos were available it's
possible to get empty output back which would cause
exception from Lua side. This patch fixes such behaviour.

Signed-off-by: Igor Shishkin <me@teran.ru>

deduplicator.lrdevplugin/FindDuplicates.lua

@@ -73,17 +73,19 @@ function FindDuplicates()
   local s = assert(f:read('*a'))
   f:close()
 
-  local imgsum_output = json:decode(s)
-
-  if imgsum_output["duplicates"] ~= nil then
-    catalog:withWriteAccessDo("Create collection", function()
-      for _, photo in pairs(imgsum_output["duplicates"]) do
-        for _, file in pairs(photo) do
-          p = catalog:findPhotoByPath(file)
-          collection:addPhotos({p})
+  if s ~= "" then
+    local imgsum_output = json:decode(s)
+
+    if imgsum_output["duplicates"] ~= nil then
+      catalog:withWriteAccessDo("Create collection", function()
+        for _, photo in pairs(imgsum_output["duplicates"]) do
+          for _, file in pairs(photo) do
+            p = catalog:findPhotoByPath(file)
+            collection:addPhotos({p})
+          end
         end
-      end
-    end)
+      end)
+    end
   end
 end