feat(builtin.treesitter) send user message if no symbols found.

[xieyonn]

Description

• Remove redundant empty check in builtin.treesitter.
• Send user a INFO message if found no symbols, just like how other builtin pickers works.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Manual testing - open a empty file and open ":Telescope treesitter"
• Manual testing - open example file a.lua and open ":Telescope treesitter"
  a.lua

var a = {}

Configuration:

• Neovim version (nvim --version):
  NVIM v0.10.4
  Build type: Release
  LuaJIT 2.1.171348406
• Operating system and version:
  Debian 12

Checklist:

• My code follows the style guidelines of this project (stylua)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (lua annotations)

[juliancoffee]

Just wanted to make the same MR, yeah, that chunk of code has a kind of confusing behaviour.