Allow adding empty column to turn off empty-value semantics

gtfs/spec/en/reference.md

@@ -466,7 +466,7 @@ To process the cost of a leg:
 |  `from_timeframe_group_id` | Foreign ID referencing `timeframes.timeframe_group_id` | Optional |  Defines the timeframe for the fare validation event at the start of the fare leg.<br><br>The “start time” of the fare leg is the time at which the event is scheduled to occur.  For example, the time could be the scheduled departure time of a bus at the start of a fare leg where the rider boards and validates their fare. For the rule matching semantics below, the start time is computed in local time, as determined by [Local Time Semantics](#localtimesemantics) of [timeframes.txt](#timeframestxt).  The stop or station of the fare leg’s departure event should be used for timezone resolution, where appropriate.<br><br>For a fare leg rule that specifies a `from_timeframe_group_id`, that rule will match a particular leg if there exists at least one record in `timeframes.txt` where all of the following conditions are true<br>- The value of `timeframe_group_id` is equal to the `from_timeframe_group_id` value.<br>- The set of days identified by the record’s `service_id` contains the “current day” of the fare leg’s start time.<br>- The “time-of-day” of the fare leg's start time is greater than or equal to the record’s `timeframes.start_time` value and less than the `timeframes.end_time` value.<br><br>An empty `fare_leg_rules.from_timeframe_group_id` indicates that the start time of the leg does not affect the matching of this rule. |
 |  `to_timeframe_group_id` |  Foreign ID referencing `timeframes.timeframe_group_id` | Optional |  Defines the timeframe for the fare validation event at the end of the fare leg.<br><br>The “end time” of the fare leg is the time at which the event is scheduled to occur.  For example, the time could be the scheduled arrival time of a bus at the end of a fare leg where the rider gets off and validates their fare.  For the rule matching semantics below, the end time is computed in local time, as determined by [Local Time Semantics](#localtimesemantics) of [timeframes.txt](#timeframestxt).  The stop or station of the fare leg’s arrival event should be used for timezone resolution, where appropriate.<br><br>For a fare leg rule that specifies a `to_timeframe_group_id`, that rule will match a particular leg if there exists at least one record in `timeframes.txt` where all of the following conditions are true<br>- The value of `timeframe_group_id` is equal to the `to_timeframe_group_id` value.<br>- The set of days identified by the record’s `service_id` contains the “current day” of the fare leg’s end time.<br>- The “time-of-day” of the fare leg's end time is greater than or equal to the record’s `timeframes.start_time` value and less than the `timeframes.end_time` value.<br><br>An empty `fare_leg_rules.to_timeframe_group_id` indicates that the end time of the leg does not affect the matching of this rule. |
 | `fare_product_id` | Foreign ID referencing `fare_products.fare_product_id` | **Required** | The fare product required to travel the leg. |
-| `rule_priority` | Non-negative integer | Optional | Defines the order of priority in which matching rules are applied to legs, allowing certain rules to take precedence over others. When multiple entries in `fare_leg_rules.txt` match, the rule or set of rules with the highest value for `rule_priority` will be selected.<br><br>An empty value for `rule_priority` is treated as zero. If the `rule_priority` field exists, it should not have all records entirely empty. |
+| `rule_priority` | Non-negative integer | Optional | Defines the order of priority in which matching rules are applied to legs, allowing certain rules to take precedence over others. When multiple entries in `fare_leg_rules.txt` match, the rule or set of rules with the highest value for `rule_priority` will be selected.<br><br>An empty value for `rule_priority` is treated as zero. |
 
 ### fare_transfer_rules.txt