Properly check if the "Context" and "Extension" headers are empty in a ShowDialPlan action.

The code which handles the ShowDialPlan action wrongly assumed that a non-NULL return value
from the function which retrieves headers from an action indicates that the header has a
value. This is incorrect and the contents must be checked to see if they are blank.

(closes issue ASTERISK-20628)
Reported by: jkroon
Patches:
     asterisk-showdialplan-incorrect-error.patch uploaded by jkroon
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Merged revisions 376166 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 376167 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 376168 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@376169 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/pbx.c

@@ -7891,17 +7891,17 @@ static int manager_show_dialplan(struct mansession *s, const struct message *m)
 
 	manager_show_dialplan_helper(s, m, idtext, context, exten, &counters, NULL);
 
-	if (context && !counters.context_existence) {
+	if (!ast_strlen_zero(context) && !counters.context_existence) {
 		char errorbuf[BUFSIZ];
 
 		snprintf(errorbuf, sizeof(errorbuf), "Did not find context %s", context);
 		astman_send_error(s, m, errorbuf);
 		return 0;
 	}
-	if (exten && !counters.extension_existence) {
+	if (!ast_strlen_zero(exten) && !counters.extension_existence) {
 		char errorbuf[BUFSIZ];
 
-		if (context)
+		if (!ast_strlen_zero(context))
 			snprintf(errorbuf, sizeof(errorbuf), "Did not find extension %s@%s", exten, context);
 		else
 			snprintf(errorbuf, sizeof(errorbuf), "Did not find extension %s in any context", exten);
@@ -7909,6 +7909,10 @@ static int manager_show_dialplan(struct mansession *s, const struct message *m)
 		return 0;
 	}
 
+	if (!counters.total_items) {
+		manager_dpsendack(s, m);
+	}
+
 	astman_append(s, "Event: ShowDialPlanComplete\r\n"
 		"EventList: Complete\r\n"
 		"ListItems: %d\r\n"