ShellPkg: Fix Shell to not return without startup.nsh after timeout

When user doesn't press key to exit the timeout waiting in Shell,
and there is no startup.nsh, Shell exits with failure status.
aaf51f08ee introduced this bug.
The patch fixes this issue.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Chen A Chen <chen.a.chen@intel.com>

ShellPkg/Application/Shell/Shell.c

@@ -1279,6 +1279,11 @@ DoStartupScript(
   if (FileStringPath != NULL) {
     Status = RunScriptFile (FileStringPath, NULL, L"", ShellInfoObject.NewShellParametersProtocol);
     FreePool (FileStringPath);
+  } else {
+    //
+    // we return success since startup script is not mandatory.
+    //
+    Status = EFI_SUCCESS;
   }
 
   return (Status);