Don't compare String8s to NULL.

Use String8::isEmpty instead. Note that this code path is hit
only if the zygote calls ::exit, and that never happens unless
the VM invocation fails.

Change-Id: I0e7d3a86a79b12b2174ca3bf0dbe1904e33c041a

cmds/app_process/app_main.cpp

@@ -97,7 +97,7 @@ public:
 
     virtual void onExit(int code)
     {
-        if (mClassName == NULL) {
+        if (mClassName.isEmpty()) {
             // if zygote
             IPCThreadState::self()->stopProcess();
         }