BUG: Fix bug in SeriesGroupBy.value_counts when DataFrame has one row (#42618)
- [x] closes #42618 
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

Was introduced in v1.3.0 [here](https://github.com/pandas-dev/pandas/pull/39326). I couldn't find a reason why `len(lchanges)` should be used and the unit tests still passed with `len(val)` instead. 