Qt turns edited locations into null-strings

They aren't null pointers after you touched them, so we can't rely on
that to choose if we should use put_string or show_utf8.
show_utf8 would not print the tag if it only contains a \0 as body.

Fixes #440

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

save-xml.c

@@ -190,7 +190,7 @@ static void show_location(struct membuffer *b, struct dive *dive)
 		int len = sprintf(buffer, "  <location ");
 
 		len += format_location(buffer + len, latitude, longitude);
-		if (!dive->location) {
+		if (!dive->location || dive->location[0] == '\0') {
 			memcpy(buffer + len, "/>\n\0", 5);
 			put_string(b, buffer);
 			return;