Skip migration if cache is empty or doesn’t exist.

Library/Homebrew/update_migrator.rb

@@ -40,6 +40,9 @@ module UpdateMigrator
 
       return if ENV.key?("HOMEBREW_DISABLE_LOAD_FORMULA")
 
+      return unless HOMEBREW_CACHE.directory?
+      return if HOMEBREW_CACHE.children.empty?
+
       ohai "Migrating cache entries..."
 
       Formula.each do |formula|
@@ -78,6 +81,9 @@ module UpdateMigrator
 
       return if ENV.key?("HOMEBREW_DISABLE_LOAD_FORMULA")
 
+      return unless HOMEBREW_CACHE.directory?
+      return if HOMEBREW_CACHE.children.empty?
+
       ohai "Migrating cache entries..."
 
       cache_entries = lambda do |path|