[DependencyInjection] Add defined prefix for env var processor

[GaryPEGEOT]

Fixes #18462

[javiereguiluz]

Thanks Gary for contributing this!

While merging, we did some tweaks: 7f31c49

1. We removed "defined" from the description because it's recommended to not include the worde being described in its own description (this is a bit confusing --> "defined [...] returns true if the env var is defined [...]")

2. We renamed the env var of the example from NO_FOO to just FOO. I see the NO_COLOR example mentioned in the code PR, but using negated env vars makes the example harder to understand.

[GaryPEGEOT]

Hi @javiereguiluz! Thanks! I think there's a small typo: #18465