backport: fix(ui2): clear empty cmdline after backspace (#41048)

Problem:
Cmdline area shows stale ":" after backspacing out of the command line.

Solution:
Clear the command line for empty commands. Note that `:<CR>` will now
clear the command line too.

(cherry picked from commit 1dbc766fa7)

test/functional/ui/cmdline2_spec.lua

@@ -275,6 +275,22 @@ describe('cmdline2', function()
       {16::}%{15:s}{16:/.\{//}^ }                                          |
     ]])
   end)
+
+  it('is empty after backspace', function()
+    feed(':')
+    screen:expect([[
+                                                           |
+      {1:~                                                    }|*12
+      {16::}^                                                    |
+    ]])
+
+    feed('<BS>')
+    screen:expect([[
+      ^                                                     |
+      {1:~                                                    }|*12
+                                                           |
+    ]])
+  end)
 end)
 
 describe('cmdline2', function()