[SchemaDumper] Hhandle tables without primary keys

(AKA:  "The Martin H. Factor: Part 2")

So this should never happen with a Rails generated table, but if you
were to create a table manually like so:

    CREATE TABLE aaa (id serial);

This wouldn't have a PRIMARY KEY associated with it.  However, Rails
always defines a PRIMARY KEY when generating it's tables:

https://github.com/rails/rails/blob/5-2-stable/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb#L214-L253

So this shouldn't ever actually happen unless you execute custom table
creates.... which apparently some people do...

lib/manageiq/schema/schema_dumper.rb

@@ -19,7 +19,7 @@ def table(table, stream)
       def add_id_column_comment(table, stream)
         pk      = @connection.primary_key(table)
         pkcol   = @connection.columns(table).detect { |c| c.name == pk }
-        comment = pkcol.comment
+        comment = pkcol.try(:comment)
 
         return unless comment