For this configuration, the external table is being used as "time series" data which comes with it some limitations. One of those limitations is that two values are required in order to interpolate. In the case of an external table such as this example which only has 1 row, it breaks the assumptions for time series data since the only way to get two values is to assume one value as NoData and the other value being the actual value (table row). As such, there is no plan to make any changes to accommodate this use case, which does not fit into the assumption of time series data. As noted, there are several workarounds that may be considered.
