fix(align): update default_merge to work with edge caes

Details:
- After b83127c, `default_merge()`
  doesn't work with some edge cases which affects `align_strings()`.

Co-authored-by: Evgeni Chasnovski <[email protected]>

Author: pkazmier

lua/mini/align.lua

@@ -1611,19 +1611,19 @@ H.default_action_merge = function(parts, opts)
 
   -- Precompute combination strings (recycle `merge` array)
   local dims = parts.get_dims()
-  local delimiter_arr = {}
+  local delim_arr = {}
   for j = 1, dims.col - 1 do
-    delimiter_arr[j] = H.slice_mod(delimiter, j)
+    delim_arr[j] = H.slice_mod(delimiter, j)
   end
 
   -- Do not change indentation
   local first_parts_are_indent = true
   for i = 1, dims.row do
     first_parts_are_indent = first_parts_are_indent and H.is_whitespace(parts[i][1])
   end
-  delimiter_arr[1] = first_parts_are_indent and delimiter_arr[1]:gsub('^%s*', '') or delimiter_arr[1]
+  delim_arr[1] = (first_parts_are_indent and delim_arr[1] ~= nil) and delim_arr[1]:gsub('^%s*', '') or delim_arr[1]
 
-  return vim.tbl_map(function(row) return H.concat_array(row, delimiter_arr) end, parts)
+  return vim.tbl_map(function(row) return H.concat_array(row, delim_arr) end, parts)
 end
 
 -- Work with modifiers --------------------------------------------------------

tests/test_align.lua

@@ -442,6 +442,13 @@ T['align_strings()']['works with multibyte characters'] = function()
   )
 end
 
+T['align_strings()']['works in edge cases'] = function()
+  validate_align_strings({}, {}, {})
+  validate_align_strings({ '' }, {}, { '' })
+  validate_align_strings({ '', '' }, {}, { '', '' })
+  validate_align_strings({ '', ' ' }, {}, { '', ' ' })
+end
+
 T['align_strings()']['does not affect input array'] = function()
   child.lua([[strings = { 'a=b', 'aa=b' }]])
   child.lua([[pre_split = { MiniAlign.new_step('aaa', function(s, _) s[1] = 'xxx' end) }]])
@@ -953,6 +960,13 @@ T['gen_step']['default_merge()']['works'] = function()
   )
 end
 
+T['gen_step']['default_merge()']['works in edge cases'] = function()
+  validate_align_strings({}, {}, {})
+  validate_align_strings({ '' }, {}, { '' })
+  validate_align_strings({ '', '' }, {}, { '', '' })
+  validate_align_strings({ '', ' ' }, {}, { '', ' ' })
+end
+
 T['gen_step']['default_merge()']['verifies relevant options'] = function()
   expect.error(
     function() child.lua([[MiniAlign.align_strings({ 'a' }, { merge_delimiter = 1 }, {})]]) end,