fix(ipc): Clear content if no output is produced
fix(ipc): Clear content if no output is produced

Before, if the command produced no output, the `m_output` field would
not have been overwritten and the old output was displayed.

But since this is an explicit trigger of the hook, the user would expect
the output to be updated to whatever the script produces (even if that
is nothing).

Ref: https://www.reddit.com/r/Polybar/comments/e9a8ww

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