`pygettext` generates output even when there should be none

Bug report

Bug description:

This occurs in two cases:

• Output is still generated when there is no input. Whereas xgettext:

  $ xgettext
  xgettext: no input file given
  Try 'xgettext --help' for more information.
  

  And the second case follows on from this (but can also occur with input files)
• In xgettext, no output is generated if there are no messages. In pygettext output is still generate even if no messages are found, the header is still sent.
  We should fix it to work like xgettext and not produce "empty" output.

cc @tomasr8

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-156322

從 pygettext 的命令列進入點開始，將無輸入和無訊息時的行為與 xgettext 進行比較。確認這兩種情況都不會產生輸出，然後檢查現有的 pygettext 測試，或為這兩種情境新增涵蓋；連結的 PR gh-156322 表明相關工作可能已經在進行中。