Incorrect display of empty HTML article body fixed.

[pboguslawski]

Proposed change

Znuny incorrectly displays articles with empty HTML bodies (error screen is displayed in the article iframe instead of empty body).

Example email message displayed incorrectly in Znuny:

From test@localhost  Tue Jan 11 09:35:34 2022
Content-Type: multipart/mixed; boundary="------------0E7SVvKDzD4zuBIUOmJEz90l"
Message-ID: <20220111001@localhost>
Date: Tue, 11 Jan 2022 09:35:31 +0100
MIME-Version: 1.0
From: test <test@localhost>
Subject: test
To: otrs@localhost
Content-Language: pl-PL

This is a multi-part message in MIME format.
--------------0E7SVvKDzD4zuBIUOmJEz90l
Content-Type: multipart/alternative;
 boundary="------------Ykl0SZabNei1x2Mr2CwcTukE"

--------------Ykl0SZabNei1x2Mr2CwcTukE
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain; charset="UTF-8"

=0D

--------------Ykl0SZabNei1x2Mr2CwcTukE
Content-Transfer-Encoding: quoted-printable
Content-Type: text/html; charset="UTF-8"

--------------Ykl0SZabNei1x2Mr2CwcTukE--

--------------0E7SVvKDzD4zuBIUOmJEz90l
Content-Type: text/plain; charset=windows-1250;
 name="test.txt"
Content-Disposition: attachment; filename="test.txt"
Content-Transfer-Encoding: 8bit

test

--------------0E7SVvKDzD4zuBIUOmJEz90l--

This mod fixes the above issue.

Type of change

• '1 - 🐞 bug 🐞' - Bugfix (non-breaking change which fixes an issue)

Additional information

Replaces: https://github.com/znuny/Znuny/pull/187 Author-Change-Id: IB#1114565

Checklist

• [x] The code change is tested and works locally.(❗)
• [x] There is no commented out code in this PR.(❕)
• [ ] You improved or added new unit tests.(❕)
• [x] Local ZnunyCodePolicy run passes successfully.(❕)
• [x] Local unit tests pass.(❕)
• [ ] GitHub workflow ZnunyCodePolicy passes.(❗)
• [ ] GitHub workflow unit tests pass.(❗)