Um, no. please don't think in terms of conditional searches in splunk, there's no such thing that you want to attempt. (Map is a method, but it's expensive and almost always the wrong approach.)
You didn't include your code, so we don't know whether you are doing a base search that gets all 4 and then filtering, or what. I will assume not.
You only want one record, so just append one default zero record onto the end of your results, then | head 1 . If there were no results, you will get your default zero record, otherwise you will get the actual record. Done.
your search that creates either 1 or zero records for counter="Express"
| append [|makeresults | eval counter="Express", Percentage=0 | table counter Percentage]
| head 1
If you have a single base search that gives all four, then you do this, with the same result
your search that creates either 1 or zero records for counter="Express"
| append [|makeresults | eval counter=mvappend("Express","High","Medium","Low"), Percentage=0 | mvexpand counter | table counter Percentage]
| dedup counter
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