Data value incorrectly shown for Lines with missing value set to Nothing

[flamber]

Describe the bug
Data point values incorrectly shown at the X-axis for Lines with missing value set to Nothing.
It essentially behaves similar to setting the line missing value to Zero.

To Reproduce

1. Native query > Sample Dataset

select 1 as "lag", 0.1 as "Jan_2021", 0.2 as "Feb_2021"
union all
select 3, 0.1, null

2. Change Visualization to Line chart, with "lag" as X-axis and the two other columns as Y-axis
3. Under Display, enable "Show values on data points" and set "Replace missing values with" to Nothing
4. Notice the two data point zeros showing up in the bottom
   

Information about your Metabase Installation:
Tested 0.38.0 thru 0.40.1

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