device: binding lookup should return null for unsupported names

A null device name should map to a null device.  So should a name that
is empty.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>

include/device.h

@@ -313,7 +313,8 @@ struct device {
  * it can use this function to retrieve the device structure of the lower level
  * driver by the name the driver exposes to the system.
  *
- * @param name device name to search for.
+ * @param name device name to search for.  A null pointer, or a pointer to an
+ * empty string, will cause NULL to be returned.
  *
  * @return pointer to device structure; NULL if not found or cannot be used.
  */

kernel/device.c

@@ -79,6 +79,13 @@ const struct device *z_impl_device_get_binding(const char *name)
 {
 	const struct device *dev;
 
+	/* A null string identifies no device.  So does an empty
+	 * string.
+	 */
+	if ((name == NULL) || (*name == 0)) {
+		return NULL;
+	}
+
 	/* Split the search into two loops: in the common scenario, where
 	 * device names are stored in ROM (and are referenced by the user
 	 * with CONFIG_* macros), only cheap pointer comparisons will be