core: call dynamic_user_acquire() only when 'group' is non-null

core: call dynamic_user_acquire() only when 'group' is non-null

When unit is reloaded, and the reloaded unit has bad-setting, then
unit_patch_contexts() is not called and exec_context::user and group
may not be configured.

A minimum reproducer for the case is:
- step 1.
$ sudo systemctl edit --full hoge.service
[Service]
oneshot
ExecStart=sleep 1h

- step 2.
$ sudo systemctl start hoge.service

- step 3.
$ sudo systemctl edit --full hoge.service
[Service]
Type=oneshot
ExecStart=@bindir@/sleep 1h
DynamicUser=yes

Then pid1 crashed.

Fixes #14733.