options: list correct default value for --screenshot-template

Make the default value part of the option metadata, instead of doing
this in the screenshot code. Makes more sense with --list-options and
the command.c option metadata properties.

options/options.c

@@ -778,6 +778,7 @@ const struct MPOpts mp_default_opts = {
     .sub_fix_timing = 1,
     .sub_cp = "auto",
     .mkv_subtitle_preroll_secs = 1.0,
+    .screenshot_template = "shot%n",
 
     .hwdec_codecs = "h264,vc1,wmv3",
 

player/screenshot.c

@@ -129,7 +129,7 @@ static char *create_fname(struct MPContext *mpctx, char *template,
     struct tm *local_time = localtime(&raw_time);
 
     if (!template || *template == '\0')
-        template = "shot%n";
+        return NULL;
 
     for (;;) {
         char *next = strchr(template, '%');