User resource with forcelocal and groups attributes set will fail if /etc/group contains empty lines

Puppet Version: 7.4.0 and 6.21.0 (and later)
Puppet Server Version: Masterless to demonstrate issue
OS Name/Version: CentOS 7

A user resource with forcelocal => true and groups attributes set will fail if /etc/group contains any empty/blank lines

# egrep -n '^$|testuser' /etc/passwd /etc/group

/etc/passwd:24:testuser:x:1001:1001:testuser:/home/testuser:/bin/bash

/etc/group:41:centos:x:1000:testuser

/etc/group:42:testuser:x:1001:

/etc/group:43:

# grpck -r

invalid group file entry

delete line ''? No

grpck: no changes

# bundle exec puppet apply -e "user { 'testuser': groups => ['centos'], forcelocal => true}"

Notice: Compiled catalog for [redacted] in environment production in 0.02 seconds

Error: undefined method `split' for nil:NilClass

#

This is due to this line in localgroup method added for PUP-10857 in puppet 6.21.0 / puppet 7.4.0 and later

This does not occur in puppet 6.19.1 and earlier (example output from 6.16 agent)

# puppet --version

6.16.0

# puppet apply -e "user { 'testuser': groups => ['centos'], forcelocal => true}"

Notice: Compiled catalog for [redacted] in environment production in 0.04 seconds

Notice: Applied catalog in 0.06 seconds

#

While empty lines in /etc/group are technically not valid configuration, the behaviour change is difficult to diagnose as it occurs when the catalog is applied/enforced, and does not return much detail in the error.

Desired Behavior:

Ideally do not fail when empty lines are encountered, or at least output more detailed failure message.

Actual Behavior:

Error: undefined method `split' for nil:NilClass