weston-launch: Only pass non-NULL value into setenv()

getenv() can return NULL is the key is not set, passing NULL into
setenv() is an error

src/weston-launch.c

@@ -515,6 +515,7 @@ setup_session(struct weston_launch *wl)
 
 	term = getenv("TERM");
 	clearenv();
+	if (term)
 		setenv("TERM", term, 1);
 	setenv("USER", wl->pw->pw_name, 1);
 	setenv("LOGNAME", wl->pw->pw_name, 1);