sysusers: make sure to reset the returned value when EOF is reached in fget*ent_sane() wrappers (#8737)

To indicate that the there're no more entries, these wrappers return false but
did leave the passed pointed unmodified.

However EOF is not an error and is a very common case so initialize the output
argument to NULL even in this case so callers don't need to do that.

Fixes: #8721

test/TEST-21-SYSUSERS/test-12.expected-group

@@ -0,0 +1,2 @@
+root:x:0:
+systemd-coredump:x:1:

test/TEST-21-SYSUSERS/test-12.expected-passwd

@@ -0,0 +1,2 @@
+root:x:0:0:root:/root:/bin/bash
+systemd-coredump:x:1:1:systemd Core Dumper:/:/sbin/nologin

test/TEST-21-SYSUSERS/test-12.initial-group

@@ -0,0 +1 @@
+root:x:0:

test/TEST-21-SYSUSERS/test-12.initial-passwd

@@ -0,0 +1 @@
+root:x:0:0:root:/root:/bin/bash

test/TEST-21-SYSUSERS/test-12.input

@@ -0,0 +1 @@
+u systemd-coredump  1 "systemd Core Dumper"