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

```
lib/ruby_header_parser/parser.rb:238:58: [error] Type `(::String | nil)` does not have method `gsub`
│ Diagnostic ID: Ruby::NoMethod
│
└           definition[0...definition.index(function_name)].gsub("char *", "char*").strip
                                                            ~~~~
```

Author: sue445

lib/ruby_header_parser/parser.rb

@@ -235,7 +235,8 @@ def parse_typeref_type(definition:, function_name:, typeref_field:, filepath:, l
           typeref_field.gsub(/[A-Z_]+\s*\(\(.*\)\)/, "").gsub("RUBY_SYMBOL_EXPORT_BEGIN", "")
         else
           # parse typeref in definition
-          definition[0...definition.index(function_name)].gsub("char *", "char*").strip
+          type = definition[0...definition.index(function_name)] || ""
+          type.gsub("char *", "char*").strip
         end
 
       typeref_type = Util.sanitize_type(typeref_type)