[lldb] Fix macOS build by replacing nullptr with FileSpec()

Before we had a implicit conversion from nullptr to FileSpec
which was thankfully removed.

lldb/source/Host/macosx/objcxx/Host.mm

@@ -1130,7 +1130,7 @@ static Status LaunchProcessPosixSpawn(const char *exe_path,
   // --arch <ARCH> as part of the shell invocation
   // to do that job on OSX.
 
-  if (launch_info.GetShell() == nullptr) {
+  if (launch_info.GetShell() == FileSpec()) {
     // We don't need to do this for ARM, and we really shouldn't now that we
     // have multiple CPU subtypes and no posix_spawnattr call that allows us
     // to set which CPU subtype to launch...

lldb/source/Symbol/LocateSymbolFileMacOSX.cpp

@@ -595,7 +595,7 @@ bool Symbols::DownloadObjectAndSymbolFile(ModuleSpec &module_spec,
         }
         Status error = Host::RunShellCommand(
             command.GetData(),
-            NULL,            // current working directory
+            FileSpec(),      // current working directory
             &exit_status,    // Exit status
             &signo,          // Signal int *
             &command_output, // Command output