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
diff --git a/src/weston-launch.c b/src/weston-launch.c
index 5b03094..7264f7e 100644
--- a/src/weston-launch.c
+++ b/src/weston-launch.c
@@ -515,7 +515,8 @@
 
 	term = getenv("TERM");
 	clearenv();
-	setenv("TERM", term, 1);
+	if (term)
+		setenv("TERM", term, 1);
 	setenv("USER", wl->pw->pw_name, 1);
 	setenv("LOGNAME", wl->pw->pw_name, 1);
 	setenv("HOME", wl->pw->pw_dir, 1);