Prevent "usermod: no changes" notice

[janw-me]

I see this notice very often, including in issues as part of debugging.
It also triggered on all of my dynamic tooling.
So rather than just silencing the error I wrapped it in an if to see if it's even needed in the first place.

I saw it every dynamic event. rough example for lando composer install

events:
  pre-composer:
    - appserver: echo DO THINGS

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[pirog]

@janw-me finally at a point in our repo reorg where this is possible although it lives in a different repo now. Going to close this but ive applied your commit over here lando/core#51