Hello 
Personally, I don’t understand what is the sense of posting empty but I can accept that there may be communities where this is common and I remember this is not the first request for something like this. 
There is a way to bypass the posting limit. It can be with for example [wrap] tags because it will be count as characters.
I know it’s probably not the best option to always type it. So I’ve made a theme component for it. I hope it will enough for you and your community too.
The theme component will add a button to the composer toolbar (by default it’s in the popup menu). Clicking on the button a placeholder text will be adds to the textarea what you can publish. This text will be hidden after you post.
The component contains some settings where you can set up 
Also contains Theme Translation section where you can customize the button name and the text what will be added the textarea…
Let’s see in action
The button is in the popup menu by default as you see in the video but it can be placed in toolbar too. To achieve this just simply disable this setting.
And than with this setting you can choose which section of the toolbar you would like to place the button.
insertions
fontStyles
extras
