say.c: Prevent erroneous failures with 'say' family of functions.

A regression was introduced in ASTERISK~29531 that caused 'say'
functions to fail with file lists that would previously have
succeeded. This caused affected channels to hang up where previously
they would have continued.

We now explicitly check for the empty string to restore the previous
behavior.

ASTERISK-29859 #close

Change-Id: Ia2e5769868e2792313c2d7c07996efe009c6f8d5

main/say.c

@@ -180,6 +180,13 @@ static int say_filenames(struct ast_channel *chan, const char *ints, const char
 	if (!filenames) {
 		return -1;
 	}
+
+	/* No filenames to play? Return success so we don't hang up erroneously */
+	if (ast_str_strlen(filenames) == 0) {
+		ast_free(filenames);
+		return 0;
+	}
+
 	files = ast_str_buffer(filenames);
 
 	while ((fn = strsep(&files, "&"))) {