shared/install: fix DefaultInstance expansion in %i

We should substitute DefaultInstance if the instance is not specified.

Test case:
[Install]
DefaultInstance=bond1
WantedBy= foobar-i-%i.device

$ systemctl --root=/ enable testing4@.service
Created symlink /etc/systemd/system/foobar-i-bond1.device.wants/testing4@bond1.service
 → /etc/systemd/system/testing4@.service.
(before, the symlink would be created as
/etc/systemd/system/foobar-i-.device.wants/testing4@bond1.service)

Fixes #4411.

src/shared/install-printf.c

@@ -27,6 +27,7 @@
 #include "install.h"
 #include "macro.h"
 #include "specifier.h"
+#include "string-util.h"
 #include "unit-name.h"
 #include "user-util.h"
 
@@ -47,7 +48,7 @@ static int specifier_prefix(char specifier, void *data, void *userdata, char **r
 }
 
 static int specifier_instance(char specifier, void *data, void *userdata, char **ret) {
-        UnitFileInstallInfo *i = userdata;
+        const UnitFileInstallInfo *i = userdata;
         char *instance;
         int r;
 
@@ -57,8 +58,8 @@ static int specifier_instance(char specifier, void *data, void *userdata, char *
         if (r < 0)
                 return r;
 
-        if (!instance) {
-                instance = strdup("");
+        if (isempty(instance)) {
+                instance = strdup(i->default_instance ?: "");
                 if (!instance)
                         return -ENOMEM;
         }