input: Write a function to convert an HTML string to plain text without using any libraries. If the HTML string is empty, return an empty string.
  assert html_to_text("<p>Hello, <b>World</b>!</p>") == "Hello, World!"
  assert html_to_text("<h1>Heading</h1>") == "Heading"
  assert html_to_text("<div><p>Text</p></div>") == "Text"
  assert html_to_text("") == ""
  assert html_to_text("<br>") == ""
output: def html_to_text(html):
      if not html:
          return ""
      text = ""
      inside_tag = False
      for char in html:
          if char == '<':
              inside_tag = True
          elif char == '>':
              inside_tag = False
          elif not inside_tag:
              text += char
      return text.strip()