core: don't reset /dev/console if stdin/stdout/stderr as passed as fd in a transient service

Otherwise we might end resetting /dev/console all the time when a transient service starts or stops.

Fixes #2377
Fixes #2198
Fixes #2061

src/basic/terminal-util.c

@@ -726,9 +726,7 @@ bool tty_is_vc_resolve(const char *tty) {
 }
 
 const char *default_term_for_tty(const char *tty) {
-        assert(tty);
-
-        return tty_is_vc_resolve(tty) ? "TERM=linux" : "TERM=vt220";
+        return tty && tty_is_vc_resolve(tty) ? "TERM=linux" : "TERM=vt220";
 }
 
 int fd_columns(int fd) {