HTMLDisable for Matrix adds empty format and formatted_body fields

[bkil]

Describe the bug
If we enable HTMLDisable, messages will not be visible for the miniVector Android Matrix client. Upon closer inspection of the event JSON, the text message is encapsulated in body, but the pruned format and formatted_body fields are not removed, they are included with an empty string value.

To Reproduce
Steps to reproduce the behavior:

• Set up a bridge between Matrix and something else (like IRC)
• Join the Matrix room using miniVector (may also affect other clients)
• Enable HTMDisable on the matrix gateway
• Send a message from IRC
• Observe that miniVector shows a message without text, can also be verified when clicking ... -> View Source on the message in Element Web

Expected behavior
If we disable HTML formatting, the fields format and formatted_body should be removed completely.

• See: https://matrix.org/docs/spec/client_server/r0.5.0#m-text

Environment (please complete the following information):

• OS: Linux
• Matterbridge version: 1.21.0 cf13fff

Additional context

[irc.myirc]
Server="chat.freenode.net:6697"
Nick="nick"
UseTLS=true

[matrix.mymatrix]
Server="https://matrix.org"
Login="nick"
Password="password"
HTMLDisable=true #### ####

[[gateway]]
name="gateway1"
enable=true

[[gateway.inout]]
account="irc.myirc"
channel="#htmldisabletest"

[[gateway.inout]]
account="matrix.mymatrix"
channel="#htmldisabletest:matrix.org"