Test whether there is a prefix for a markov name.

When the prefix is empty, there is no name so directly return.

This with commit f941c70081 makes it
possible to undo recruitment of undead units.

This breaks compatibility so is not intended for backporting.

changelog

@@ -83,6 +83,7 @@ Version 1.13.0-dev:
    * Fix bug 21459 by making dropped sides default to idle.
    * Fix bug 21882 by introducing "fight_on_without_leader" boolean attribute
      of [side] tags and refactoring check_victory to use this.
+   * Changed: Don't use the random generator for units with no names.
 
 Version 1.11.11:
  * Add-ons server:

src/race.cpp

@@ -67,8 +67,9 @@ static markov_prefix_map markov_prefixes(const std::vector<std::string>& items,
 static ucs4::string markov_generate_name(const markov_prefix_map& prefixes,
 	size_t chain_size, size_t max_len)
 {
-	if(chain_size == 0)
+	if(prefixes.empty() || chain_size == 0) {
 		return ucs4::string();
+	}
 
 	ucs4::string prefix, res;