On demand generation of boot files

Change-Id: I7ba2a3cb627e1dc53410a4d3e67364550bdd995b
diff --git a/src/space.cc b/src/space.cc
index ddbf068..73701cd 100644
--- a/src/space.cc
+++ b/src/space.cc
@@ -279,7 +279,7 @@
 }
 
 ImageSpace* Space::CreateImageSpace(const std::string& image_file_name) {
-  CHECK(image_file_name != NULL);
+  CHECK(!image_file_name.empty());
 
   uint64_t start_time = 0;
   if (VLOG_IS_ON(heap) || VLOG_IS_ON(startup)) {