dyndnsd: handle potential nil cases detected by sorbet

- including review suggestions from @jgraichen

lib/dyndnsd/database.rb

@@ -15,7 +15,7 @@ module Dyndnsd
     # @return [void]
     def load
       if File.file?(@db_file)
-        @db = JSON.parse(File.open(@db_file, 'r', &:read))
+        @db = JSON.parse(File.read(@db_file, mode: 'r'))
       else
         @db = {}
       end

lib/dyndnsd/helper.rb

@@ -57,7 +57,7 @@ module Dyndnsd
           'error.kind': e.class.to_s,
           'error.object': e,
           message: e.message,
-          stack: e.backtrace.join("\n")
+          stack: e.backtrace&.join("\n") || ''
         )
         raise
       ensure

lib/dyndnsd/updater/command_with_bind_zone.rb

@@ -14,7 +14,7 @@ module Dyndnsd
       # @return [void]
       def update(db)
         Helper.span('updater_update') do |span|
-          span.set_tag('dyndnsd.updater.name', self.class.name.split('::').last)
+          span.set_tag('dyndnsd.updater.name', self.class.name&.split('::')&.last || 'None')
 
           # write zone file in bind syntax
           File.open(@zone_file, 'w') { |f| f.write(@generator.generate(db)) }
@@ -24,7 +24,7 @@ module Dyndnsd
           end
 
           # detach so children don't become zombies
-          Process.detach(pid)
+          Process.detach(pid) if pid
         end
       end
     end