v4l2: set default standard to NULL

Avoid a failure with the default value of "NTSC". Not all drivers
support a standard.

libavdevice/v4l2.c

@@ -717,7 +717,7 @@ static int v4l2_read_close(AVFormatContext *s1)
 #define DEC AV_OPT_FLAG_DECODING_PARAM
 
 static const AVOption options[] = {
-    { "standard", "", OFFSET(standard), FF_OPT_TYPE_STRING, {.str = "NTSC" }, 0, 0, AV_OPT_FLAG_DECODING_PARAM },
+    { "standard", "", OFFSET(standard), FF_OPT_TYPE_STRING, {.str = NULL }, 0, 0, AV_OPT_FLAG_DECODING_PARAM },
     { "channel",  "", OFFSET(channel),  FF_OPT_TYPE_INT,    {.dbl = 0 }, 0, INT_MAX, AV_OPT_FLAG_DECODING_PARAM },
     { "video_size", "A string describing frame size, such as 640x480 or hd720.", OFFSET(video_size), FF_OPT_TYPE_STRING, {.str = NULL}, 0, 0, DEC },
     { NULL },