DataService. _performAndDeleteOfflineOperation incorrectly null checks for offlineService and schema

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The logic of the following section of DataService. _performAndDeleteOfflineOperation is logically inconsistent.

            if (this.offlineService) {
                tableSchema = this.offlineService.schema[operationType];
                foreignKeys = tableSchema.foreignKeys;
            }

            if (!foreignKeys) {
                foreignKeys = tableSchema._computedForeignKeys ||
                    (tableSchema._computedForeignKeys = Object.keys(operation.changes));
            }

https://github.com/montagejs/montage/blob/master/data/service/data-service.js#L2395-L2404

If this.offlineService is undefined, tableSchema and foreignKeys will not be set in the first conditional. That forces the code to throw in the second condition. !foreignKeys is true so the code enters the block and tableSchema is necessarily undefined.

[tejaede]

@marchant

I am not sure what the correct solution is so this issue is designed to track the discussion. This is not a blocker for us.