Cover the empty-match guard in Paragraph#substitute

A mutation check showed no spec failed when the guard against empty
matches was removed, although without it a pattern like /x*/ loops
forever. Pin the existing behavior, under a timeout so a regression
fails instead of hanging.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: satoryu

spec/docx/containers/paragraph_spec.rb

@@ -2,6 +2,7 @@
 
 require 'spec_helper'
 require 'docx'
+require 'timeout'
 
 describe Docx::Elements::Containers::Paragraph do
   def paragraph(xml, document_properties = {}, document = nil)
@@ -142,6 +143,19 @@ def paragraph(xml, document_properties = {}, document = nil)
       expect(p.text).to eq('{{a}}{{a}}')
     end
 
+    # Without the guard against empty matches this would loop forever, so it
+    # runs under a timeout to fail instead of hanging.
+    it 'ignores a pattern that only matches the empty string' do
+      p = paragraph('<w:p><w:r><w:t>abc</w:t></w:r></w:p>')
+
+      Timeout.timeout(5) do
+        p.substitute(/x*/, '-')
+        p.substitute(/x*/, '')
+      end
+
+      expect(p.text).to eq('abc')
+    end
+
     it 'leaves the paragraph untouched when nothing matches' do
       expect(split.substitute('{{missing}}', 'x').text).to eq('Hello {{first_name}}!')
     end