fix: stats1 null_count with --fr regex gives wrong results (#1994)

When using --fr (regex field selector) with stats1 -a null_count, void
(empty) field values were unconditionally skipped in
ingestWithValueFieldRegexes, causing null_count to always report 0.

The non-regex path (ingestWithoutValueFieldRegexes) already had a special
case that allows void values through for null_count accumulators. This
commit adds the same exception to the regex path.

Fixes #1639

Co-authored-by: cobyfrombrooklyn-bot <cobyfrombrooklyn-bot@users.noreply.github.com>

pkg/transformers/stats1.go

@@ -578,7 +578,9 @@ func (tr *TransformerStats1) ingestWithValueFieldRegexes(
 				// The accumulator has been initialized with default values;
 				// continue here. (If we were to continue outside of this loop
 				// we would be failing to construct the accumulator.)
-				continue
+				if accumulatorName != "null_count" {
+					continue
+				}
 			}
 			namedAccumulator.Ingest(valueFieldValue)
 		}

test/cases/verb-stats1-regexed-field-names/0008/cmd

@@ -0,0 +1 @@
+mlr --icsv --ifs "|" --oxtab stats1 -a null_count --fr ".*" test/input/null-count-pipe.csv

test/cases/verb-stats1-regexed-field-names/0008/expout

@@ -0,0 +1,4 @@
+Id_null_count         0
+Type_null_count       3
+Other Type_null_count 1
+Number_null_count     0

test/input/null-count-pipe.csv

@@ -0,0 +1,4 @@
+Id|Type|Other Type|Number
+a||foo|1
+b||bar|2
+c|||1