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.",

src/shared/install.c

@@ -340,9 +340,12 @@ void install_changes_dump(int r, const char *verb, const InstallChange *changes,
         assert(verb || r >= 0);
 
         for (size_t i = 0; i < n_changes; i++) {
-                if (changes[i].type < 0)
-                        assert(verb);
                 assert(changes[i].path);
+                /* This tries to tell the compiler that it's safe to use 'verb' in a string format if there
+                 * was an error, but the compiler doesn't care and fails anyway, so strna(verb) is used
+                 * too. */
+                assert(verb || changes[i].type >= 0);
+                verb = strna(verb);
 
                 /* When making changes here, make sure to also change install_error() in dbus-manager.c. */