remove duplicated logic by defining the method to read log files

test/command_line/helper/command_runner.rb

@@ -189,7 +189,7 @@ def run_command_interactive(*command_line,
           rescue SystemCallError
           end
         end
-        error_output = @error_output_log_path.read.encode("UTF-8", "locale")
+        error_output = read_log(@error_output_log_path)
         Result.new("", error_output)
       end
     end
@@ -201,8 +201,8 @@ def run_command_sync(*command_line)
       :err => @error_output_log_path.to_s,
     }
     succeeded = system(*command_line, options)
-    output = @output_log_path.read.encode("UTF-8", "locale")
-    error_output = @error_output_log_path.read.encode("UTF-8", "locale")
+    output = read_log(@output_log_path)
+    error_output = read_log(@error_output_log_path)
     unless succeeded
       message = <<-MESSAGE.chomp
 failed to run: #{command_line.join(" ").encode("UTF-8")}
@@ -217,4 +217,9 @@ def run_command_sync(*command_line)
     end
     Result.new(output, error_output)
   end
+
+  def read_log(log_path)
+    return "" unless File.exist?(log_path)
+    File.read(@log_path).encode("UTF-8", "locale")
+  end
 end