`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

Comienza en el punto de entrada de línea de comandos de pygettext y compara su comportamiento sin entrada y sin mensajes con xgettext. Confirma que ambos casos no producen ninguna salida y, a continuación, revisa las pruebas existentes de pygettext o añade cobertura para estos dos escenarios; el PR vinculado gh-156322 indica que es posible que ya se esté trabajando en ello.