[FIX] web_editor: properly save media dialog without any image
[FIX] web_editor: properly save media dialog without any image

Commit https://github.com/odoo/odoo/commit/50418c481182cb414792da32f2a0529503e59f9c
was correctly forward-ported up to 11.0. This commit fixes the
forward-port from saas-11.2.

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