fix: don't return empty hover

Nvim show me an error(since:
neovim/neovim#34789), when I hover an symbol
whose definition cannot be found by luals
```
**/neovim/runtime/lua/vim/lsp/util.lua:1685: 'width' key must be a
positive Integer
```

To reprodunce, hover on a with this file
```lua
---@type a
```

Haven't reproduced with other lsp but I think it's harmless to PR here.

Author: phanen

script/provider/provider.lua

@@ -369,9 +369,13 @@ m.register 'textDocument/hover' {
         if not hover or not source then
             return nil
         end
+        local value = tostring(hover)
+        if #value == 0 then
+            return nil
+        end
         return {
             contents = {
-                value = tostring(hover),
+                value = value,
                 kind  = 'markdown',
             },
             range = converter.packRange(state, source.start, source.finish),