sd-device: never return NULL+0

It is highly confusing if a getter function returns 0, but the value is
set to NULL. This, right now, triggers assertions as code relies on the
returned values to be non-NULL.

Like with sd-bus-creds and friends, return 0 only if a value is actually
available.

Discussed with Tom, and actually fixes real bugs as in #512.

src/libsystemd/sd-device/sd-device.c

@@ -791,6 +791,9 @@ _public_ int sd_device_get_subsystem(sd_device *device, const char **ret) {
                 device->subsystem_set = true;
         }
 
+        if (!device->subsystem)
+                return -ENOENT;
+
         *ret = device->subsystem;
 
         return 0;
@@ -908,6 +911,9 @@ _public_ int sd_device_get_driver(sd_device *device, const char **ret) {
                         return log_debug_errno(r, "sd-device: could not set driver for %s: %m", device->devpath);
         }
 
+        if (!device->driver)
+                return -ENOENT;
+
         *ret = device->driver;
 
         return 0;
@@ -1002,6 +1008,8 @@ _public_ int sd_device_get_sysname(sd_device *device, const char **ret) {
                         return r;
         }
 
+        assert_return(device->sysname, -ENOENT);
+
         *ret = device->sysname;
 
         return 0;