Fix NilClass error when configuring previous Lockbox master keys (#3950)

Assign previous_versions directly instead of appending, preventing
NoMethodError when PWPUSH_MASTER_KEY_PREVIOUS is set.

Signed-off-by: Martin Stenröse <martin@stenrose.se>

Author: martinstenrose

config/initializers/lockbox.rb

@@ -26,7 +26,7 @@
 Lockbox.master_key = ENV.fetch("PWPUSH_MASTER_KEY", "749b1022e1cb83fb04f3022eacaf3bfef60c6d47f83e6fb41f534a05fc69929f")
 
 if ENV.key?("PWPUSH_MASTER_KEY_PREVIOUS")
-  ENV.fetch("PWPUSH_MASTER_KEY_PREVIOUS").split(",").each do |previous_key|
-    Lockbox.default_options[:previous_versions] << {master_key: previous_key}
+  Lockbox.default_options[:previous_versions] = ENV.fetch("PWPUSH_MASTER_KEY_PREVIOUS").split(",").map do |previous_key|
+    {master_key: previous_key}
   end
 end