install: fix compiler warning about empty directive argument

install: fix compiler warning about empty directive argument

On ppc64el with gcc 13.2 on Ubuntu 24.04:

3s In file included from ../src/basic/macro.h:386,
483s                  from ../src/basic/alloc-util.h:10,
483s                  from ../src/shared/install.c:12:
483s ../src/shared/install.c: In function ‘install_changes_dump’:
483s ../src/shared/install.c:432:64: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
483s   432 |                         err = log_error_errno(changes[i].type, "Failed to %s unit, unit %s does not exist.",
483s       |                                                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
483s ../src/shared/install.c:432:75: note: format string is defined here
483s   432 |                         err = log_error_errno(changes[i].type, "Failed to %s unit, unit %s does not exist.",