Fixed [error] Type (::String | nil) does not have method length

```
lib/ruby_header_parser/parser.rb:357:29: [error] Type `(::String | nil)` does not have method `length`
│ Diagnostic ID: Ruby::NoMethod
│
└       ::Regexp.last_match(1).length
                               ~~~~~~
```

Author: sue445

lib/ruby_header_parser/parser.rb

@@ -354,7 +354,7 @@ def prepare_argument_parts(parts:, arg_pos:)
     # @param type [String]
     def pointer_length(type)
       type =~ /(\*+)$/
-      ::Regexp.last_match(1).length
+      ::Regexp.last_match(1)&.length || 0
     end
   end
 end