Fix #1639: stats1 null_count with --fr regex gives wrong results

[cobyfrombrooklyn-bot]

Problem

When using --fr (regex field selector) with stats1 -a null_count, void (empty) field values are always reported as 0, even when there are actual empty fields. Using -f with explicit field names correctly counts them.

Example from #1639:

# With -f: correctly reports Type_null_count=3
mlr --icsv --ifs "|" stats1 -a null_count -f "Type" input.csv

# With --fr: incorrectly reports Type_null_count=0
mlr --icsv --ifs "|" stats1 -a null_count --fr ".*" input.csv

Root Cause

In stats1.go, ingestWithoutValueFieldRegexes (the non-regex path) has a special case at line 532 that allows void values through for null_count accumulators:

if valueFieldValue.IsVoid() {
    if accumulatorName != "null_count" {
        continue
    }
}

But ingestWithValueFieldRegexes (the regex path) unconditionally skips void values:

if valueFieldValue.IsVoid() {
    continue
}

Fix

Added the same null_count exception to the regex path, making it consistent with the non-regex path.

Test

Added regression test case test/cases/verb-stats1-regexed-field-names/0008 with a pipe-delimited CSV containing empty fields. Verifies that --fr ".*" produces the same null_count results as explicit -f. All 49 stats1-related regression tests pass.

Tested locally on macOS ARM (Apple Silicon).

[johnkerl]

Thanks!!!!!