Fix a Ruby::AnnotationSyntaxError

The following changes caused the error.
- soutaro/steep#1384

```
lib/lrama/state/reduce.rb:30:36: [error] Type annotation has a syntax error: Failed to parse a type in annotation
│ Diagnostic ID: Ruby::AnnotationSyntaxError
│
└           # @type ivar @look_ahead: Array<Grammar::Symbol>
                                      ~~~~~~~~~~~~~~~~~~~~~~
```

lib/lrama/state/reduce.rb

@@ -9,7 +9,7 @@ class Reduce
 
       def initialize(item)
         @item = item
-        @look_ahead = nil
+        @look_ahead = []
         @not_selected_symbols = []
       end
 
@@ -26,8 +26,7 @@ def add_not_selected_symbol(sym)
       end
 
       def selected_look_ahead
-        if @look_ahead
-          # @type ivar @look_ahead: Array<Grammar::Symbol>
+        if @look_ahead.any?
           @look_ahead - @not_selected_symbols
         else
           []

sig/lrama/state/reduce.rbs

@@ -2,11 +2,11 @@ module Lrama
   class State
     class Reduce
       @item: States::Item
-      @look_ahead: Array[Grammar::Symbol]?
+      @look_ahead: Array[Grammar::Symbol]
       @not_selected_symbols: Array[Grammar::Symbol]
 
       attr_reader item: States::Item
-      attr_reader look_ahead: Array[Grammar::Symbol]?
+      attr_reader look_ahead: Array[Grammar::Symbol]
       attr_reader not_selected_symbols: Array[Grammar::Symbol]
       attr_accessor default_reduction: bool