diagnostic: skip xcode-select check with no Xcode.

Skip the `xcode-select` configuration check if there's no CLT or Xcode
installed as in that case neither will be used.

Fixes #1055.

Library/Homebrew/extend/os/mac/diagnostic.rb

@@ -233,6 +233,7 @@ module Homebrew
 
       def check_xcode_select_path
         return if MacOS::CLT.installed?
+        return unless MacOS::Xcode.installed?
         return if File.file?("#{MacOS.active_developer_dir}/usr/bin/xcodebuild")
 
         path = MacOS::Xcode.bundle_path