Custom Expression filter not setting the "Done" button to current state of the formula until onblur

[flamber]

Describe the bug
The Done-button on Custom Expression can be somewhat difficult to figure out, when it's possible to finish, since it sometimes requires blurring the input before the state gets updated.

To Reproduce

1. Custom question > Sample Dataset > Products
2. Filter > Custom Expression - notice the Done-button is enabled by default
   
3. Blur the input - now the Done-button is disabled
   
4. Type 1=1 in the formula - the Done-button is still disabled. Pressing enter does not make a difference, still disabled. But it's possible to click the disabled button, since it will blur the input and enable the button instantly.
   

Expected behavior
The Done-button should not be enabled on empty formula.
And perhaps should follow the state of the formula.

Information about your Metabase Installation:
Tested 0.36.8 thru 0.38.2 and master

[flamber]

Fixed by #15293