imprv: Print less undesired new lines in the protocol debug console
This is not a full fix, but a workaround that improves the console usability on noisy accounts.
When filtering out the existing buffer, the undesired new lines remain, but I don't think this is fixable without a large refactor, possibly without using a SourceView. This MR improves the situation for stanzas printed after the filter has been set. The drawback here is that filtered stanzas are completely dropped when filters are active, so it's not possible to see "unhide" them anymore.
References: #11497