rtp: fix bad option default

NULL does not map to an entry in the choice list.

also, mark the choice labels for translation.

modules/access/rtp/rtp.c

@@ -539,8 +539,8 @@ error:
     "(between 96 and 127) if it can't be determined otherwise with " \
     "out-of-band mappings (SDP)" )
 
-static const char *const dynamic_pt_list[] = { "theora" };
-static const char *const dynamic_pt_list_text[] = { "Theora Encoded Video" };
+static const char *const dynamic_pt_list[] = { "", "theora" };
+static const char *const dynamic_pt_list_text[] = { N_( "Unset" ), N_( "Theora Encoded Video" ) };
 
 /*
  * Module descriptor
@@ -580,7 +580,7 @@ vlc_module_begin()
     add_integer("rtp-max-misorder", 100, RTP_MAX_MISORDER_TEXT,
                 RTP_MAX_MISORDER_LONGTEXT)
         change_integer_range (0, 32767)
-    add_string("rtp-dynamic-pt", NULL, RTP_DYNAMIC_PT_TEXT,
+    add_string("rtp-dynamic-pt", "", RTP_DYNAMIC_PT_TEXT,
                RTP_DYNAMIC_PT_LONGTEXT)
         change_string_list(dynamic_pt_list, dynamic_pt_list_text)