fix: check if static_file_r_path is set

lib/jekyll-multiple-languages-plugin.rb

@@ -34,12 +34,16 @@ module Jekyll
       static_files.delete_if do |static_file|
 
         # Remove "/" from beginning of static file relative path
-        static_file_r_path    = static_file.instance_variable_get(:@relative_path).dup
-        static_file_r_path[0] = ''
-
-        exclude_paths.any? do |exclude_path|
-          Pathname.new(static_file_r_path).descend do |static_file_path|
-            break(true) if (Pathname.new(exclude_path) <=> static_file_path) == 0
+        if static_file.instance_variable_get(:@relative_path) != nil
+          static_file_r_path = static_file.instance_variable_get(:@relative_path).dup
+          if static_file_r_path
+            static_file_r_path[0] = ''
+
+            exclude_paths.any? do |exclude_path|
+              Pathname.new(static_file_r_path).descend do |static_file_path|
+                break(true) if (Pathname.new(exclude_path) <=> static_file_path) == 0
+              end
+            end
           end
         end
       end