app_read: Fix custom terminator functionality regression

Currently, when the t option is specified with no arguments,
the # character is still treated as a terminator, even though
no character should be treated as a terminator.

This is because a previous regression fix was modified to
remove the use of NULL as a default altogether. However,
NULL and an empty string actually refer to different
arrangements and should be treated differently. NULL is the
default terminator (#), while an empty string removes the
terminator altogether. This is the behavior being used by
the rest of the core.

Additionally, since S_OR catches empty strings as well as
NULL (not intended), this is changed to a ternary operator
instead, which fixes the behavior.

ASTERISK-29705 #close

Change-Id: I9b6b72196dd04f5b1e0ab5aa1b0adf627725e086

apps/app_read.c

@@ -250,7 +250,7 @@ static int read_exec(struct ast_channel *chan, const char *data)
 						break;
 					}
 					tmp[x++] = res;
-					if (strchr(terminator, tmp[x-1])) {
+					if (terminator && strchr(terminator, tmp[x-1])) {
 						tmp[x-1] = '\0';
 						status = "OK";
 						break;

main/app.c

@@ -249,7 +249,7 @@ enum ast_getdata_result ast_app_getdata_terminator(struct ast_channel *c, const
 			fto = 50;
 			to = ast_channel_pbx(c) ? ast_channel_pbx(c)->dtimeoutms : 2000;
 		}
-		res = ast_readstring(c, s, maxlen, to, fto, S_OR(terminator, "#"));
+		res = ast_readstring(c, s, maxlen, to, fto, (terminator ? terminator : "#"));
 		if (res == AST_GETDATA_EMPTY_END_TERMINATED) {
 			return res;
 		}