fix(charclass): make behavior with empty str match latest Vim (#19749)

Later Vim patches changed to return 0 for empty string and null string.
Also update setcellwidth() docs to match latest Vim.

src/nvim/testdir/test_functions.vim

@@ -1774,6 +1774,8 @@ func Test_charclass()
   call assert_equal(1, charclass('.'))
   call assert_equal(2, charclass('x'))
   call assert_equal(3, charclass("\u203c"))
+  " this used to crash vim
+  call assert_equal(0, "xxx"[-1]->charclass())
 endfunc
 
 func Test_eventhandler()