fix(options): disallow empty 'fdc' and 'scl' (#16765)

Empty string values for these options aren't actually allowed, but
check_opt_strings allows empty string options.

It so happens that 'scl' handles empty string like "auto", but empty 'fdc'
causes glitchiness (win_fdccol_count returns an incorrect value).

Just disallow empty string values for these options completely.

test/functional/legacy/options_spec.lua

@@ -83,4 +83,9 @@ describe('set', function()
       Press ENTER or type command to continue^   |
     ]])
   end)
+
+  it('foldcolumn and signcolumn to empty string is disallowed', function()
+    matches('E474: Invalid argument: fdc=', exc_exec('set fdc='))
+    matches('E474: Invalid argument: scl=', exc_exec('set scl='))
+  end)
 end)