rebuild: more solid check for str-or-empty

scripts/rebuild.rkt

@@ -18,7 +18,7 @@
     (exit 1)))
 
 (define (str-or-empty hash key str)
-  (if (hash-has-key? hash key) str ""))
+  (if (hash-ref hash key #f) str ""))
 
 (define (higan-make args)
   (sys (~a "make -j12 -C " args " uninstall"))