Handle the case when XML is empty

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What does this MR do and why?

Issue: #268035 (closed)

There is a problem with Gitlab::XmlConverter that we recently introduced in !148779 (merged): if the XML string is empty, an error will be reported.

Currently MR fixes this problem.

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Before	After

How to set up and validate locally

# Before
[1] Gitlab::XmlConverter.new('')
NoMethodError: undefined method `to_hash` for nil:NilClass
from /Users/deer/.asdf/installs/ruby/3.2.3/lib/ruby/gems/3.2.0/gems/activesupport-7.0.8.1/lib/active_support/xml_mini/nokogiri.rb:36:in `to_hash`

# After
[1] pry(main)> Gitlab::XmlConverter.new('')
=> #<Gitlab::XmlConverter:0x0000000299adf6b0>
[2] pry(main)> Gitlab::XmlConverter.new('').to_h
=> nil