markup_parser: skip parsing HTML content when it is empty (#161)

## Issue

This PR resolves the following CI error.

```
Error:
FullTextSearch::BatchRunnerSynchronizeTest#test_archived_changeset:
RuntimeError: input string cannot be empty
    plugins/full_text_search/lib/full_text_search/markup_parser.rb:20:in `parse'
    plugins/full_text_search/lib/full_text_search/issue_mapper.rb:25:in `upsert_fts_target'
    plugins/full_text_search/lib/full_text_search/batch_runner.rb:129:in `block (2 levels) in synchronize_fts_targets_internal'
    plugins/full_text_search/lib/full_text_search/batch_runner.rb:335:in `each'
    plugins/full_text_search/lib/full_text_search/batch_runner.rb:335:in `each'
    plugins/full_text_search/lib/full_text_search/batch_runner.rb:335:in `iterate'
    plugins/full_text_search/lib/full_text_search/batch_runner.rb:121:in `block in synchronize_fts_targets_internal'
    plugins/full_text_search/lib/full_text_search/resolver.rb:43:in `each'
    plugins/full_text_search/lib/full_text_search/resolver.rb:43:in `each'
    plugins/full_text_search/lib/full_text_search/batch_runner.rb:116:in `synchronize_fts_targets_internal'
    plugins/full_text_search/lib/full_text_search/batch_runner.rb:12:in `synchronize'
    plugins/full_text_search/test/unit/full_text_search/batch_runner_synchronize_test.rb:34:in `setup'

bin/rails test plugins/full_text_search/test/unit/full_text_search/batch_runner_synchronize_test.rb:219
```

## Cause

The error occurs because Nokogiri's behavior for handling an empty
string changed in version 1.17.0.

- Before 1.17.0: `Nokogiri::HTML::SAX::Parser#parse` returned `nil` when
passed an empty string.
- After 1.17.0: `Nokogiri::HTML::SAX::Parser#parse` raises a
`RuntimeError` when passed an empty string.

Refer to the following Nokogiri commit for details:
sparklemotion/nokogiri@35596e7

## Solution

Skip parsing the HTML content when it is empty by adding a presence
check.

---------

Co-authored-by: Sutou Kouhei <[email protected]>

Author: otegami

lib/full_text_search/markup_parser.rb

@@ -15,6 +15,8 @@ def parse(object, attribute, options={})
       html = with_user(User.admin.first) do
         textilizable(object, attribute, options)
       end
+      return ["", []] unless html.present?
+
       document = Document.new
       parser = Nokogiri::HTML::SAX::Parser.new(document)
       parser.parse(html)