fix(native_comp): symlink creation was missing a conditional check

Latest commits to master as of June 26, 2021 have moved *.eln files back
to: MacOS/lib/emacs/**/native-lisp

Even though the symlinks are most likely no longer needed, I don't know
of an easy way to determine if they're required or not, so for now,
they'll be created to ensure older native-comp branch SHAs can still be
built.

build-emacs-for-macos

@@ -420,7 +420,10 @@ class Build
 
       source = Dir['MacOS/libexec/emacs/**/eln-cache',
                    'MacOS/lib/emacs/**/native-lisp'].first
-      err 'Failed to find native-lisp cache directory for symlink creation.'
+
+      if source.nil?
+        err 'Failed to find native-lisp cache directory for symlink creation.'
+      end
 
       target = File.basename(source)
       FileUtils.ln_s(source, target) unless File.exist?(target)