Teach copy_string to return NULL on empty string

In subsurface we store no value as a NULL pointer everywhere, but
sometimes some structures returns a empty field as a empty string.

This teaches our helper copy_string to return NULL if you try to copy a
empty string.

This fixes a bug where we store buddy and divemaster in git as empty
strings if they passed via MainTab::saveTaggedStrings().

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

dive.h

@@ -32,7 +32,7 @@ static inline int same_string(const char *a, const char *b)
 
 static inline char *copy_string(const char *s)
 {
-	return s ? strdup(s) : NULL;
+	return (s && *s) ? strdup(s) : NULL;
 }
 
 #include <libxml/tree.h>