make proposal table_id nullable

database/migrations/2018_07_08_194000_create_proposals_table.php

@@ -19,7 +19,7 @@ class CreateProposalsTable extends Migration
 
             // note that a revision may be shared by multiple tables, thus a proposal may also apply to different tables
 
-            $table->unsignedInteger('table_id')->index(); // table this proposal was written for
+            $table->unsignedInteger('table_id')->index()->nullable(); // table this proposal was written for
             $table->unsignedInteger('revision_id')->index(); // parent revision (applying it to a different revisions may cause conflicts)
             $table->unsignedInteger('author_id')->index();