feat: add utils.empty() function for checking empty values

This commit introduces a new utility function `empty()` that checks if a value
is empty. This function handles nil values, empty tables, and whitespace-only
strings.

The function is used to improve empty value checking across the codebase,
replacing direct nil checks with the more robust empty() function.

Changes:
- Add utils.empty() function
- Update client.lua to use empty() for response validation
- Update chat.lua to use empty() for content checks

Author: deathbeam

lua/CopilotChat/client.lua

@@ -659,7 +659,7 @@ function Client:ask(prompt, opts)
   end
 
   if is_stream then
-    if not full_response then
+    if utils.empty(full_response) then
       for _, line in ipairs(vim.split(response.body, '\n')) do
         parse_stream_line(line)
       end
@@ -668,7 +668,7 @@ function Client:ask(prompt, opts)
     parse_line(response.body)
   end
 
-  if not full_response then
+  if utils.empty(full_response) then
     error('Failed to get response: empty response')
     return
   end

lua/CopilotChat/ui/chat.lua

@@ -345,7 +345,7 @@ function Chat:parse_history()
 
   local history = {}
   for _, section in ipairs(self.sections) do
-    if section.content then
+    if not utils.empty(section.content) then
       if section.answer then
         table.insert(history, {
           content = section.content,

lua/CopilotChat/utils.lua

@@ -459,4 +459,23 @@ function M.key_to_info(name, key, surround)
   return out
 end
 
+--- Check if a value is empty
+---@param v any The value
+---@return boolean
+function M.empty(v)
+  if not v then
+    return true
+  end
+
+  if type(v) == 'table' then
+    return vim.tbl_isempty(v)
+  end
+
+  if type(v) == 'string' then
+    return vim.trim(v) == ''
+  end
+
+  return false
+end
+
 return M