fix(exports): display exact number of dossiers count at time generation

app/models/export.rb

@@ -51,6 +51,7 @@ class Export < ApplicationRecord
   end
 
   def compute
+    self.dossiers_count = dossiers_for_export.count
     load_snapshot!
 
     file.attach(blob.signed_id) # attaching a blob directly might run identify/virus scanner and wipe it
@@ -109,9 +110,10 @@ class Export < ApplicationRecord
   end
 
   def count
-    if procedure_presentation_id.present?
-      dossiers_for_export.count
-    end
+    return dossiers_count if !dossiers_count.nil? # export generated
+    return dossiers_for_export.count if procedure_presentation_id.present?
+
+    nil
   end
 
   def procedure