Fix hphpd to not raise a notice when no file is given at the command line

"hhvm -m debug" without specifying a file should work without raising
notices. hphp_invoke_simple() and friends used to (incorrectly) tolerate
using an empty string for the file name, and hphpd was relying on this.

This diff fixes hphpd to not call hphp_invoke_simple() when no file was
given at the command line.

hphp/runtime/eval/debugger/debugger.cpp

@@ -143,7 +143,9 @@ void Debugger::DebuggerSession(const DebuggerClientOptions& options,
     DebuggerDummyEnv dde;
     Debugger::InterruptSessionStarted(file.c_str());
   }
-  hphp_invoke_simple(file);
+  if (!file.empty()) {
+    hphp_invoke_simple(file);
+  }
   {
     DebuggerDummyEnv dde;
     Debugger::InterruptSessionEnded(file.c_str());