lib: bt_version_get_extra(): return NULL if none instead of empty str.

This follows the pattern we have for other optional strings returned by
the library.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I775f4f3be917bde405ad3b5e63183dae9609cf03
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2833
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>

include/babeltrace2/version.h

@@ -92,13 +92,7 @@ This extra information can contain a version suffix such as
 <code>-pre5</code> or <code>-rc1</code>.
 
 @returns
-    @parblock
-    Extra information about the library's version.
-
-    Cannot be \c NULL.
-
-    Can be an empty string if there's no extra information.
-    @endparblock
+    Extra information about the library's version, or \c NULL if none.
 */
 extern const char *bt_version_get_extra(void);
 

src/lib/babeltrace2.c

@@ -24,6 +24,7 @@
 
 #include <babeltrace2/babeltrace.h>
 #include <stdlib.h>
+#include <string.h>
 
 unsigned int bt_version_get_major(void)
 {
@@ -41,5 +42,5 @@ unsigned int bt_version_get_patch(void) {
 
 const char *bt_version_get_extra(void)
 {
-	return BT_VERSION_EXTRA;
+	return strlen(BT_VERSION_EXTRA) == 0 ? NULL : BT_VERSION_EXTRA;
 }