is_dir should be false for false

This is breaking Symphony

hphp/runtime/ext/ext_file.cpp

@@ -742,6 +742,9 @@ bool f_is_file(CStrRef filename) {
 
 bool f_is_dir(CStrRef filename) {
   String cwd;
+  if (filename.empty()) {
+    return false;
+  }
   bool isRelative = (filename.charAt(0) != '/');
   if (isRelative) cwd = g_context->getCwd();
   if (!isRelative || cwd == RuntimeOption::SourceRoot.c_str()) {

hphp/test/quick/is_dir.php

@@ -0,0 +1,6 @@
+<?php
+
+var_dump(is_dir(false));
+var_dump(is_dir(true));
+var_dump(is_dir(""));
+var_dump(is_dir(null));

hphp/test/quick/is_dir.php.expect

@@ -0,0 +1,4 @@
+bool(false)
+bool(false)
+bool(false)
+bool(false)