fix(extension): fix some extension showing a section in whole line

For now use a dummy component to lualine_x on extensions that only define
lualine_a. This will let the renderer know right sections do exist.

closes #73

lua/lualine/extensions/fzf.lua

@@ -4,10 +4,18 @@ local function fzf_statusline()
   return 'FZF'
 end
 
+local empty = {
+  function()
+    return ' '
+  end,
+  padding = 0,
+}
+
 local M = {}
 
 M.sections = {
   lualine_a = { fzf_statusline },
+  lualine_x = { empty },
 }
 
 M.filetypes = { 'fzf' }

lua/lualine/extensions/nerdtree.lua

@@ -4,10 +4,18 @@ local function get_short_cwd()
   return vim.fn.fnamemodify(vim.fn.getcwd(), ':~')
 end
 
+local empty = {
+  function()
+    return ' '
+  end,
+  padding = 0,
+}
+
 local M = {}
 
 M.sections = {
   lualine_a = { get_short_cwd },
+  lualine_x = { empty },
 }
 
 M.filetypes = { 'nerdtree' }

lua/lualine/extensions/toggleterm.lua

@@ -5,10 +5,18 @@ local function toggleterm_statusline()
   return 'ToggleTerm #' .. vim.b.toggle_number
 end
 
+local empty = {
+  function()
+    return ' '
+  end,
+  padding = 0,
+}
+
 local M = {}
 
 M.sections = {
   lualine_a = { toggleterm_statusline },
+  lualine_x = { empty },
 }
 
 M.filetypes = { 'toggleterm' }