timeseries() expression missing sum() results for empty buckets

See solr-user post https://lists.apache.org/thread.html/aeacef8fd8cee980bb74f2f6b7e1c3fd0b7ead7d7a0e7b79dd48659f@%3Csolr-user.lucene.apache.org%3E

We have a timeseries expression with gap="+1DAY" and a sum(imps_l) to aggregate sums of an integer for each bucket. Now, some day buckets do not contain any documents at all, and instead of returning a tuple with value 0, it returns a tuple with no entry at all for the sum, see the bucket for date_dt 2018-06-22 below:

{
 "result-set": {
   "docs": [
     {
       "sum(imps_l)": 0,
       "date_dt": "2018-06-21",
       "count(*)": 5
     },
     {
       "date_dt": "2018-06-22",
       "count(*)": 0
     },
     {
       "EOF": true,
       "RESPONSE_TIME": 3
     }
   ]
 }
}

Now when we want to convert this into a column using col(a,'sum(imps_l)') then that array will get mostly numbers but also some string entries 'sum(imps_l)' which is the key name. I need purely integers in the column.

Should the timeseries() have output values for all functions even if there are no documents in the bucket? Or is there something similar to the select() expression that can take a stream of tuples not originating directly from search() and replace values? Or is there perhaps a function that can loop through the column produced by col() and replace non-numeric values with 0?