`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

从 pygettext 的命令行入口点开始，将无输入和无消息时的行为与 xgettext 进行比较。确认这两种情况下都不会产生输出，然后检查现有的 pygettext 测试，或为这两种场景添加覆盖；链接的 PR gh-156322 表明相关工作可能已经在进行中。