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

@@ -382,7 +382,7 @@ def fill_sym_to_rules
     end
 
     def validate_rule_lhs_is_nterm!
-      errors = []
+      errors = [] #: Array[String]
 
       rules.each do |rule|
         next if rule.lhs.nterm?