Fix a warning for Ruby::UnannotatedEmptyCollection

```
lib/lrama/grammar.rb:385:15: [warning] Empty array doesn't have type annotation
│ Diagnostic ID: Ruby::UnannotatedEmptyCollection
│
└       errors = []
                 ~~
```

lib/lrama/grammar.rb

@@ -382,12 +382,10 @@ def fill_sym_to_rules
     end
 
     def validate_rule_lhs_is_nterm!
-      errors = []
-
-      rules.each do |rule|
+      errors = rules.each_with_object(Array.new) do |rule, list|
         next if rule.lhs.nterm?
 
-        errors << "[BUG] LHS of #{rule.display_name} (line: #{rule.lineno}) is term. It should be nterm."
+        list << "[BUG] LHS of #{rule.display_name} (line: #{rule.lineno}) is term. It should be nterm."
       end
 
       return if errors.empty?