Empty DM view is confusing when switched to a space - find and add existing DM conversations to space

### Your use case

#### What would you like to do?

When the DM tab in a space has no DM's available (empty view), prompt and hint to switch to your "Home" space with all of your DM's. This could just be a small link text in the empty view.

And the `💭+` button should allow me to see and add existing DM's to the space. Currently, it just lists people within the space, which for my "Personal" space, is just me. This is not useful. I don't want to add more people to the space anyway, just my existing DM's to the space.

#### Why would you like to do it?

It's confusing to see none of your DM's listed when switched to a space unknowingly. Initially when none of my DM's showed up, I thought it was just a unreliable/spotty data problem while skiing up in the mountains (I even [rageshaked](https://github.com/matrix-org/element-android-rageshakes/issues/35616) about it). But after a whole day of no DM's, I finally figured out that I switched to my small "Personal" space which doesn't have any DM's in it. I just had to switch back to the `Home` space to see everything again.


### Have you considered any alternatives?

_No response_

### Additional context

Home empty screens initially added in https://github.com/vector-im/element-android/issues/2347 via https://github.com/vector-im/element-android/pull/2450