titlebar: show placeholder when no project is selected (#8021)

When no project is selected, the recent project dropdown is displaying
an empty string, making the button basically impossible to click. This
PR adds a placeholder value for that case.

Here is what it looks like now:

![image](https://github.com/zed-industries/zed/assets/53836821/c831a1eb-722e-4189-8f8b-8b3039daf8f8)



Release Notes:

- Added placeholder to titlebar when no project is selected