Fix HTMLFormatter for textarea

[APB9785]

Resolves #2838

HTMLFormatter is adding a newline between <textarea> tags, which adds unexpected whitespace to the rendered output. This can be reproduced by setting the line length on the following test:

assert_formatter_doesnt_change(
  """
  <textarea></textarea>
  """,
  line_length: 5
)

The culprit is a check for buffer != [] which fails the test when present but passes when the check is removed.

[josevalim]

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