Handle nil @syslog in syslog sink add_record

After resetting @syslog to nil between tests, some tests that use the
syslog sink without calling open() first would fail with:
"undefined method `log' for nil"

Add a guard clause to return early if @syslog is nil, preventing errors
when the sink hasn't been properly opened yet. This is a defensive check
that allows the sink to gracefully handle uninitialized state.

Author: johha

lib/steno/sink/syslog.rb

@@ -35,6 +35,7 @@ def open(identity)
 
   def add_record(record)
     return if record.log_level == :off
+    return unless @syslog
 
     record = truncate_record(record)
     msg = @codec.encode_record(record)

spec/spec_helper.rb

@@ -207,9 +207,7 @@
 
     rspec_config.after do
       # Reset the Syslog sink singleton to prevent mock leakage between tests
-      if defined?(Steno::Sink::Syslog)
-        Steno::Sink::Syslog.instance.instance_variable_set(:@syslog, nil)
-      end
+      Steno::Sink::Syslog.instance.instance_variable_set(:@syslog, nil) if defined?(Steno::Sink::Syslog)
     end
 
     rspec_config.after(:each, type: :legacy_api) { add_deprecation_warning }