dyndnsd: handle potential nil cases detected by sorbet

- including review suggestions from @jgraichen

lib/dyndnsd.rb

@@ -96,8 +96,14 @@ module Dyndnsd
 
       # drop priviliges as soon as possible
       # NOTE: first change group than user
-      Process::Sys.setgid(Etc.getgrnam(config['group']).gid) if config['group']
-      Process::Sys.setuid(Etc.getpwnam(config['user']).uid) if config['user']
+      if config['group']
+        group = Etc.getgrnam(config['group'])
+        Process::Sys.setgid(group.gid) if group
+      end
+      if config['user']
+        user = Etc.getpwnam(config['user'])
+        Process::Sys.setuid(user.uid) if user
+      end
 
       setup_traps
 

lib/dyndnsd/database.rb

@@ -14,7 +14,7 @@ module Dyndnsd
 
     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

@@ -42,7 +42,7 @@ 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

@@ -11,7 +11,7 @@ module Dyndnsd
 
       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)) }
@@ -21,7 +21,7 @@ module Dyndnsd
           end
 
           # detach so children don't become zombies
-          Process.detach(pid)
+          Process.detach(pid) if pid
         end
       end
     end