Always inherit environment when launching external terminal (#1340)

Follow up to #1318

The intention of that change was to honor environment variables sent by the
debug adapter, but it also unintentionally changed behavior to no longer
inherit the parent environment if no `env` option was provided by the debug
adapter.

This fixes it, so that:

- No `env` provided: Use parent environment
- `env` provided: Merge with parent environment. (`env` having higher priority)

lua/dap/session.lua

@@ -118,11 +118,15 @@ local function launch_external_terminal(env, terminal, args)
   local full_args = {}
   vim.list_extend(full_args, terminal.args or {})
   vim.list_extend(full_args, args)
-  local env_formatted = {}
-  -- Copy environment, prefer vars set by client
-  for k, v in pairs(env and vim.tbl_extend('keep', env, vim.fn.environ()) or {}) do
-    if k:find "^[^=]*$" then -- correct variable?
-      env_formatted[#env_formatted+1] = k.."="..tostring(v)
+  -- Initializing to nil is important so environment is inherited by the terminal
+  local env_formatted = nil
+  if env then
+    env_formatted = {}
+    -- Copy environment, prefer vars set by client
+    for k, v in pairs(vim.tbl_extend('keep', env, vim.fn.environ())) do
+      if k:find "^[^=]*$" then -- correct variable?
+        env_formatted[#env_formatted+1] = k.."="..tostring(v)
+      end
     end
   end
   local opts = {