chan_sip: Set up calls without audio (text+video), again.

The previous commit 6d980de2 fixed this issue in the core of Asterisk.
With that, each channel technology can be used without audio
theoretically. Practically, the channel-technology driver chan_sip
turned out to have an invalid check preventing that. chan_sip tested
whether there is at least one audio format. However, chan_sip has to
test whether there is at least one format. More cannot be tested while
requesting chan_sip because only the [general] capabilities but not the
[peer] caps are known yet. And the [peer] caps might not be a subset or
show any intersection with the [general] caps. This change here fixes
this.

The original commit f04d5fb8, thirteen years ago, contained a software
bug as it passed ANY audio capability to the channel-technology driver.
Instead, it should have passed NO audio format. Therefore, this
addressed issue here was not noticed in Asterisk 1.6.x and Asterisk 1.8.
Then, Asterisk 10 changed that from ANY to NO, but nobody reported since
then.

ASTERISK-29265

Change-Id: Ic16a3bf13cd1b5c4fc4041ed74961177d96b600f

channels/chan_sip.c

@@ -30796,7 +30796,6 @@ static struct ast_channel *sip_request_call(const char *type, struct ast_format_
 	char *ext = NULL, *host;
 	char tmp[256];
 	struct ast_str *codec_buf = ast_str_alloca(AST_FORMAT_CAP_NAMES_LEN);
-	struct ast_str *cap_buf = ast_str_alloca(AST_FORMAT_CAP_NAMES_LEN);
 	char *dnid;
 	char *secret = NULL;
 	char *md5secret = NULL;
@@ -30812,17 +30811,8 @@ static struct ast_channel *sip_request_call(const char *type, struct ast_format_
 		AST_APP_ARG(remote_address);
 	);
-	/* mask request with some set of allowed formats.
-	 * XXX this needs to be fixed.
-	 * The original code uses AST_FORMAT_AUDIO_MASK, but it is
-	 * unclear what to use here. We have global_capabilities, which is
-	 * configured from sip.conf, and sip_tech.capabilities, which is
-	 * hardwired to all audio formats.
-	 */
-	if (!(ast_format_cap_has_type(cap, AST_MEDIA_TYPE_AUDIO))) {
-		ast_log(LOG_NOTICE, "Asked to get a channel of unsupported format %s while capability is %s\n",
-			ast_format_cap_get_names(cap, &codec_buf),
-			ast_format_cap_get_names(sip_cfg.caps, &cap_buf));
+	if (ast_format_cap_empty(cap)) {
+		ast_log(LOG_NOTICE, "Asked to get a channel without offering any format\n");
 		*cause = AST_CAUSE_BEARERCAPABILITY_NOTAVAIL;	/* Can't find codec to connect to host */
 		return NULL;
 	}