MINOR: stick_table: centralize the handling of empty keys

Right now, it's up to each pattern fetch method to return NULL when an
empty string is returned, which is neither correct nor desirable as it
is only stick tables which need to ignore empty patterns. Let's perform
this check in stktable_fetch_key() instead.

src/stick_table.c

@@ -609,6 +609,9 @@ struct stktable_key *stktable_fetch_key(struct stktable *t, struct proxy *px, st
 	if (!static_table_key.key)
 		return NULL;
 
+	if (static_table_key.key_len == 0)
+		return NULL;
+
 	if ((static_table_key.key_len < t->key_size) && (t->type != STKTABLE_TYPE_STRING)) {
 		/* need padding with null */
 
@@ -635,7 +638,6 @@ struct stktable_key *stktable_fetch_key(struct stktable *t, struct proxy *px, st
 		memset(static_table_key.key + static_table_key.key_len, 0, t->key_size - static_table_key.key_len);
 	}
 
-
 	return &static_table_key;
 }