fix(shared): do not treat empty tables as list in deep extend (#15094)

An empty table was previously always treated as a list, which means that
while merging tables, whenever an empty table was encountered it would
  always truncate any table on the left.

`vim.tbl_deep_extend("force", { b = { a = 1 } }, { b = {} })`

Before: `{ b = {} }`
After: `{ b = { a = 1 } }`

runtime/lua/vim/shared.lua

@@ -200,6 +200,10 @@ function vim.tbl_isempty(t)
   return next(t) == nil
 end
 
+local function can_merge(v)
+  return type(v) == "table" and (vim.tbl_isempty(v) or not vim.tbl_islist(v))
+end
+
 local function tbl_extend(behavior, deep_extend, ...)
   if (behavior ~= 'error' and behavior ~= 'keep' and behavior ~= 'force') then
     error('invalid "behavior": '..tostring(behavior))
@@ -219,8 +223,8 @@ local function tbl_extend(behavior, deep_extend, ...)
     vim.validate{["after the second argument"] = {tbl,'t'}}
     if tbl then
       for k, v in pairs(tbl) do
-        if type(v) == 'table' and deep_extend and not vim.tbl_islist(v) then
+        if deep_extend and can_merge(v) and can_merge(ret[k]) then
-          ret[k] = tbl_extend(behavior, true, ret[k] or vim.empty_dict(), v)
+          ret[k] = tbl_extend(behavior, true, ret[k], v)
         elseif behavior ~= 'force' and ret[k] ~= nil then
           if behavior == 'error' then
             error('key found in more than one map: '..k)

test/functional/lua/vim_spec.lua

@@ -603,6 +603,31 @@ describe('lua stdlib', function()
       return vim.tbl_islist(c) and count == 0
     ]]))
 
+    eq(exec_lua([[
+      local a = { a = { b = 1 } }
+      local b = { a = {} }
+      return vim.tbl_deep_extend("force", a, b)
+    ]]), {a = {b = 1}})
+
+    eq(exec_lua([[
+      local a = { a = 123 }
+      local b = { a = { b = 1} }
+      return vim.tbl_deep_extend("force", a, b)
+    ]]), {a = {b = 1}})
+
+    ok(exec_lua([[
+      local a = { a = {[2] = 3} }
+      local b = { a = {[3] = 3} }
+      local c = vim.tbl_deep_extend("force", a, b)
+      return vim.deep_equal(c, {a = {[3] = 3}})
+    ]]))
+
+    eq(exec_lua([[
+      local a = { a = { b = 1} }
+      local b = { a = 123 }
+      return vim.tbl_deep_extend("force", a, b)
+    ]]), {a = 123 })
+
     eq('Error executing lua: vim/shared.lua:0: invalid "behavior": nil',
       pcall_err(exec_lua, [[
         return vim.tbl_deep_extend()